Prior to this commit, FreeMarkerView used the system default charset to
render. This commit switches this by defaulting to UTF-8, if no charset
is specified in the content type.

 - Add contentType parameter to AbstractView.renderInternal, used to
 determine the charset contained therein
 - Adds a defaultCharset property to AbstractView and
 ViewResolverSupport.

Changed View's render method from taking a HandlerResult to taking a
Map<String, ?>, in order to facilitate scenarios where a HandlerResult
is not available (i.e. web.reactive.function).

This commit fixes `ResponseEntityResultHandler` so that it only tries to
call `writeBody` if the `ResponseEntity` is not null. In case the
response entity body is null, the response is flushed right away and the
request is signaled as handled.

Issue: SPR-14663

Issue: SPR-14522

Issue: SPR-14522

Issue: SPR-14658

Issue: SPR-14646

Prior to this commit, exceptions thrown by the
`HandlerMethodArgumentResolver` would be wrapped into
`IllegalStateException`s.

This commit makes sure that a DEBUG log is written with the relevant
information and that the root cause is not wrapped into another
exception, so that the appropriate `ExceptionHandler` can be used to
deal with this error.

Issue: SPR-14618

There is really no need for a result handler dedicated to a void
return value and it's actually problematic to have it.

Each result handler treats void as necessary. For an @ResponseBody
method it means an empty body. For view resolution it means no specific
value was returned and we should procede with selecting a default view
name. Having a dedicated void result handler can interfere with this
especially since view resolution needs to be last in order.

At the same time there are cases when no result handling is needed
and the response is fully handled within the HandlerAdapter. This is
the case with WebHandler and the SimpleHandlerAdapter. For that case
we simply return mono.then(aVoid -> Mono.empty()) which effectively
returns an empty Mono and no result handling follows. The
HandlerAdapter already says you can return no values at all if the
response is fully handled.

Issue: SPR-14159

Ensure type-level Javadoc in every class, comply with guidelines for
80 char on Javadoc, and minor polish.

This commit adds support for handling an empty request body with both
HttpEntity where the body is not required and with @RequestBody where
the body is required depending on the annotation's required flag.

If the body is an explicit type (e.g. String, HttpEntity<String>) and
the body is required an exception is raised before the method is even
invoked or otherwise the body is passed in as null.

If the body is declared as an async type (e.g. Mono<String>,
HttpEntity<Mono<String>>) and is required, the error will flow through
the async type. If not required, the async type will be passed with no
values (i.e. empty).

A notable exception is rx.Single which can only have one value or one
error and cannot be empty. As a result currently the use of rx.Single
to represent the request body in any form effectively implies the body
is required.

The RequestBodyArgumentResolver has been refactored to have a shared
base class and tests with the new HttpEntityMethodArgumentResolver.

An HttpEntity argument is not expected to have an async wrapper because
the request headers are available immediately. The body however can be
asynchronous, e.g. HttpEntity<Flux<String>>.

When using the ConversionService to check and bridge to and from
reactive types we now generallly provide the full type information
available from method signatures. However that full type information
is not always necessary such as when we perform additional checks on
the generics of the reactive type (e.g. Mono<ResponseEntity>).

This allows us to switch to use DefaultFormattingConversionService
instead of GenericConversionService while also ensuring that the
CollectionToObjectConverter doesn't think it can convert List<?> to
any reactive type.

The ObjectToObjectConverter can also interfere because it is smart
enough to find the "from(Publisher<?>)" method on Flux and Mono.
To make up for that on the response side we now check if a type
is assignable to Publisher first in which case it is a simple cast.

In turn that means we don't need a PublisherToFluxConverter which can
be problematic in its own right because it can convert from Mono to
Flux which technically doesn't lose data but switches stream semantics.

Issue: #124, #128

HandlerAdapter's should always be able to provide a MethodParameter
which in turn ensures that HandlerResultHandler's have full type
information from method declarations.

This commit also introduces ResolvableMethod for use in tests to make
it easy to obtain MethodParameter return types.

Issue: #128

This commit ensures stream semantics (Flux vs Mono) are adhered to also
on the target side.

This commit adds a Decoder#decodeOne() method in order
to handle correctly the streaming versus one value
deserialization based on the type provided by the user.

For example, if a List parameter is provided in a controller
method, Jackson will be called once, while if the user provides
a Flux or an Observable parameter, Jackson will be called for
each element.

This is a port of the following commit, adapted for Java 8+:
https://github.com/spring-projects/spring-framework/commit/89396ff01ff159aa7df18e332f3888cf9ce3dc20

Currently ResponseEntityResultHandler is ordered lower than
ResponseBodyResultHandler by default whch means a ResponseEntity
should not be picked by the ResponseBodyResultHandler.

However as it is easy to have both ResponseEntity and @ResponseBody
e.g. in @RestControler (or even by mistake) and in general it makes
sense for ResponseBodyResultHandler to explicitly recognize and
ignore the ResponseEntity return type.

Before this commit only ResponseEntity with async body was supported,
e.g. ResponseEntity<Mono<String>>

This commit also adds suppport for an asyn wrapper around,
e.g. Mono<ResponseEntity<String>.

Introduce separate test classes for each base class in the hierarchy
above @ResponseBody and ResponseEntity result handlers.

Also start porting existing unit test cases for @ResponseBody and
ResponseEntity return value handlers.