Migrate all tests to consistently use BDD Mockito. Also add
checksyle rule to enforce going forwards.

Organize test imports to expand all '.*' static imports into
fully qualified imports.

This update will allow us to use additional checkstyle rules in
the future, and will also help if we migrate fully to AssertJ.

org.junit.Assert.assertThat() is deprecated as of JUnit 4.13.

See gh-22894

This commit updates URLs to prefer the https protocol. Redirects are not followed to avoid accidentally expanding intentionally shortened URLs (i.e. if using a URL shortener).

# Fixed URLs

## Fixed Success
These URLs were switched to an https URL with a 2xx status. While the status was successful, your review is still recommended.

* [ ] http://www.apache.org/licenses/ with 1 occurrences migrated to:
  https://www.apache.org/licenses/ ([https](https://www.apache.org/licenses/) result 200).
* [ ] http://www.apache.org/licenses/LICENSE-2.0 with 6515 occurrences migrated to:
  https://www.apache.org/licenses/LICENSE-2.0 ([https](https://www.apache.org/licenses/LICENSE-2.0) result 200).

This commit updates URLs to prefer the https protocol. Redirects are not followed to avoid accidentally expanding intentionally shortened URLs (i.e. if using a URL shortener).

# Fixed URLs

## Fixed Success
These URLs were switched to an https URL with a 2xx status. While the status was successful, your review is still recommended.

* [ ] http://www.apache.org/licenses/ with 1 occurrences migrated to:
  https://www.apache.org/licenses/ ([https](https://www.apache.org/licenses/) result 200).
* [ ] http://www.apache.org/licenses/LICENSE-2.0 with 6625 occurrences migrated to:
  https://www.apache.org/licenses/LICENSE-2.0 ([https](https://www.apache.org/licenses/LICENSE-2.0) result 200).

Issue: SRP-17475

Issue: SPR-16832

Most Servlet containers do this anyway, but not all, and not
consistently for forward and backslashes.

Issue: SPR-16616

MockServerWebExchange now has a second factory method that accepts a
MockServerHttpRequest.BaseBuilder.

Issue: SPR-16079

Prior to this commit, the `ResourceWebHandler` would itself handle the
response with an HTTP 404 in many cases, including a missing static
resource.

This does not give a chance to `WebExceptionHandler` instances to handle
that error and, for example, display an error page.

See spring-projects/spring-boot#8625

Issue: SPR-16023

Discovered late, but not too late. MockServerWebExchange is now in the
proper package matching to the location of ServerWebExchange.

PathContainer now has separate parsePath and parseUrlPath methods each
creating PathSegment and UrlPathSegment respectively.

Issue: SPR-15718

There is no need for ResourceWebHandler to go through the
PathExtensionContentTypeResolver when MediaTypeFactory makes it easy to
perform such lookups for a given Resource.

This does not support any extensions explicitly registered through a
WebFluxConfigurer but it would be easy enough to pass those into
ResourceWebHandler as a simple Map<String, MediaType>, should the need
arise.

Issue: SPR-15639

This commit updates the test code base to conform to changes in media
types returned by the MediaTypeFactory.

Issue: SPR-14908

Fold ResourceRegionHttpMessageWriter into ResourceHttpMessageWriter.
The latter was a private helper (not meant to be exposed) and the two
have much in common now sharing a number of private helper methods.

The combined class does not extend AbstractServerHttpMessageConverter
from which it was not using anything.

Internally the combined class now delegates directly to ResourceEncoder
or ResourceRegionEncoder as needed. The former is no longer wrapped
with EncoderHttpMessageWriter which is not required since "resource"
MediaType determination is a bit different.

The consolidation makes it easy to see the entire algorithm in one
place especially for server side rendering (and HTTP ranges). It
also allows for consistent determination of the "resource" MediaType
via MediaTypeFactory for all use cases.

ResourceRegionHttpMessageWriter no longer extends from
EncoderHttpMessageWriter freeing it to pass the correct content type
into the encoder.

Considering that the main benefit of EncoderHttpMessageWriter is to
deal with content type fallback cases, there is nothing to be missed.

Furthermore ResourceRegionHttpMessageWriter is a package private class
that is used internally within ResourceHttpMessageWriter and never
exposed externally as a an actual HttpMessageWriter.

Issue: SPR-15358

EncoderHttpMessageWriter now consistently uses the same MediaType to
set on the response and to pass to the encoder.

Issue: SPR-15357

Issue: SPR-15350

Before this change the write Publisher was saved and Mono.empty()
returned from the write metohd which did not properly implement
the write contract since no writing ("consuming") was done.

This can be a problem in some cases. For example the request may appear
to succeed even if the publisher produces an error later when
subscribed to later after request handling completes.

This commit introduces a writeHandler function in the mock request and
response. By default it "writes" by consuming the content immediately,
which allows it to return a Mono<Void> that properly reflects when
writing is done, and it also caches the data so it may be replayed
later for test assertions.

For streaming scenario a custom writeHandler may be registered which
allows the custom handling to determine how long to stream before
cancelling so request handling may complete.

Issue: SPR-14590

Issue: SPR-15190

MockServerHttpRequest and MockServerHttpResponse now extend the same
abstract base classes that server-specific implementations do and
therefore approximate their behavior more closely.

As an immediate consequence MockServerHttpRequest is read-only after
it is created. Instead it now exposes static builder methods similar
to those found in RequestEntity. This enforces more strictness as well
as recycling of requests in tests and provides nicer builder methods.

To simplify tests DefaultServerWebExchange now offers a constructor
with just a request and response, and automatically creating a
DefaultWebSessionManager.

The spring-test module now also contains client-side reactive mock
request and response implementations. The mock client request extends
the same AbstractClientHttpRequest as client-specific implementations
do. There is no abstract base class for client responses.

Issue: SPR-14590

Issue: SPR-14800

 - ScriptedSubscriber has been renamed to Verifier
 - The Publisher is passed to create() instead of verify()
 - No more need to specify the generic type explicitly
 - Version is now sync with reactor-core

Issue: SPR-14800

Reactor recently added the `ScriptedSubscriber` in its new
`reactor-addons` module. This `Subscriber` revissits the previous
`TestSubscriber` with many improvements, including:

* scripting each expectation
* builder API that guides you until the final verification step
* virtual time support

This commit refactor all existing tests to use this new
infrastructure and removed the `TestSubscriber` implementation.

Issue: SPR-14800

This commit handles range requests in `ResourceWebHandler`, using the
`ResourceHttpMessageWriter` configured within the handler.

This `WebHandler` will add a `HTTP_RANGE_REQUEST_HINT` writer hint
containing all the HTTP Range information of the request.

Issue: SPR-14664

Issue: SPR-14521

A straight-forward port of the resource handling support in
spring-webmvc to spring-web-reactive. Primarily adapting contracts and
implementations to use the reactive request and response and the
reactive ResourceHttpMessageWriter.

Issue: SPR-14521