Closes gh-23145

This commit makes sure that in DefaultMultipartMessageReader's
DefaultFilePart, the file is not closed before all bytes are written,
by using DataBufferUtils.write (see c1b6885191d6a50347aeaa14da994f0db88f26fe).

The commit also improves on the logging of the
DefaultMultipartMessageReader.

Closes gh-23130

Closes gh-23122

Closes gh-23128

Closes gh-22798

Closes gh-23117

Closes gh-23064

Closes gh-22989

Prior to this commit, MockHttpServletRequest.setCookies() produced one
Cookie header per supplied cookie, resulting in multiple Cookie headers
which violates the specification.

This commit fixes this by ensuring that all cookie name-value pairs are
stored under a single Cookie header, separated by a semicolon.

Closes gh-23074

Closes gh-23093

This commit allows to configure a custom path separator when parsing and
matching path patterns with `PathPatternParser`, but also when parsing
incoming paths as `PathContainer` instances.

Closes gh-23092

Similar to fix applied to Synchronoss implementation in #cef98e.

1. Add contentType and filename options to PartBuilder.

2. Revert recently committed #44659f since asyncPart can't properly
support Publisher of Part (only Mono, can't support filename), and
replace that with support for Part in the regular part method.

Closes gh-23083

Improve Javadoc
Consistently reject Publisher unless using asyncPart
Consistently set Content-Type when specified

Closes gh-23077

Closes gh-23051

Closes gh-23042

This commit adds a new constructor to `JettyClientHttpConnector` and
deprecates another one. Jetty is not creating `HttpClient` instances
using a builder API, but rather setting immutable configuration at
constructor time and using setters for the rest.

This commit addresses that by deprecating the constructor variant
accepting a `Consumer` and just delegating to Spring's implementation
for setting the client resources as needed.

Closes gh-22977

This commit migrates to the MockitoJUnitRunner where sensible, which
will later allow for an easier migration to Mockito's extension for
JUnit Jupiter.

In addition, this commit deletes unnecessary stubbing for various mocks
and polishes test fixture setup in various test classes.

This change is a follow-up to the ones done as part of gh-22788.

Closes gh-22954

Migrate all existing JUnit 4 `assert...` based assertions to AssertJ
and add a checkstyle rule to ensure they don't return.

See gh-23022

Migrate all existing `assertThat(..., Matcher)` assertions to AssertJ
and add checkstyle rules to ensure they don't return.

See gh-23022

This commit introduces a getOrEmpty(String) method in HttpHeaders that
returns an immutable, empty list if no values are present for the
specified header name. This is provided as a convenience over the
existing get(String) method which returns null in such cases.

Closes gh-22949

Migrate tests that use `@test(expectedException=...)` or
`try...fail...catch` to use AssertJ's `assertThatException`
instead.

Closes gh-22982

EncoderHttpMessageWriter takes advantage of the Encoder#encodeValue
that's new in 5.2 in order to produce a Mono<DataBuffer> instead of
producing a Flux<DataBuffer> and then using flux.singleOrEmpty().

Closes gh-22952

Closes gh-22976