DataBuffers::split (and underlying algorithm) should not be returning
a Flux<DataBuffer>, but rather a Flux<Flux<DataBuffer>>. In other words,
it should not join all data buffers that come before a delimiter.

Providing an implementation of a such a fully reactive split method
proved to be beyond the scope of this release, so this commit removes
the method altogether.

Due to the imminent removal of DataBufferUtils.split, this commit copies
over the buffering split algortihm from DataBufferUtils, as it is still
sutable for the StringDecoder

- Made sure that takeUntilByteCount is usable in multiple subscriptions.
- Added composite Matcher

The DefaultMultipartMessageReader has been removed for 5.2 and will be
part of a future release. This commit switches back to the
SynchronossPartHttpMessageReader.

gh-21659

See gh-23251

Closes gh-23251

Fixes gh-23253

Prior to this commit, the Basic Authentication credentials were encoded for
each request in ExchangeFilterFunctions.basicAuthentication(String, String).

This commit addresses this minor performance issue by encoding the
credentials prior to the creation of the lambda expression returned by
ExchangeFilterFunctions.basicAuthentication(String, String).

Closes gh-23256

Closes gh-23257

The new annotation helps to differentiate the handling of connection
level frames (SETUP and METADATA_PUSH) from the 4 stream requests.

Closes gh-23177

This commit renames RSocketRequester.RequestSpec data extension to
dataWithType to avoid shadowing issues and adds Publisher<T> and Flow<T>
variants to provide automatic reified type parameter resolution for
those types. It also makes RSocketRequester consistent with the
changes introduced via 2b4d6ce3 in WebFlux.

Extension functions need to use reified types to create
ParameterizedTypeReference.

Closes gh-23185

The commit deprecates syncBody(Object) in favor of body(Object)
which has the same behavior in ServerResponse, WebClient and
WebTestClient. It also adds body(Object, Class) and
body(Object, ParameterizedTypeReference) methods in order to support
any reactive type that can be adapted to a Publisher via
ReactiveAdapterRegistry. Related BodyInserters#fromProducer
methods are provided as well.

Shadowed Kotlin body<T>() extensions are deprecated in favor of
bodyWithType<T>() ones, including dedicated Publisher<T> and
Flow<T> variants. Coroutines extensions are adapted as well, and
body(Object) can now be used with suspending functions.

Closes gh-23212

Closes gh-23247

Prior to this commit, if the user configured an empty path for the
targetRequestPath property of a FlashMap, the FlashMapManager threw a
StringIndexOutOfBoundsException when saving the output FlashMap for the
next request.

This commit fixes this by skipping the decoding and normalization of an
empty target request path.

Fixes gh-23240

See gh-23237

Closes gh-23239

Closes gh-23237

Prior to Spring Framework 5.1.3, MimeTypeUtils.parseMimeTypes() and
MediaType.parseMediaTypes() ignored empty entries, but 5.1.3 introduced
a regression in that an empty entry -- for example, due to a trailing
comma in the list of media types in an HTTP Accept header -- would result
in a "406 Not Acceptable" response status.

This commit fixes this by filtering out empty entries before parsing
them into MimeType and MediaType instances. Empty entries are therefore
effectively ignored.

Fixes gh-23241

Closes gh-23207

Closes gh-23210

Closes gh-23200