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

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 commit wraps up the previous commits:

 - It uses HttpMessageConverter in the web.reactive.server package instead of Encoder/Decoder.
 - It introduces tests for the Resource @ResponseBodies.

The new package is parallel to the "mvc" package under spring-webmvc
and a place to hold the various programming model styles.

A model is created and passed to argument resolvers including a new
ModelArgumentResolver. The model is then exposed for result handling
as a field in HandlerResult.

WebServerExchange -> ServerWebExchange

Follows the same convention as in the http package also better allowing
the possibility for a client equivalent in the future.

WebToHttpHandlerBuilder -> WebHttpHandlerBuilder
WebToHttpHandlerAdapter -> WebHttpHandlerAdapter

More consistent with Spring conventions.

-> reactor.Mono : reactor.core.publisher.Mono
-> reactor.Flux : reactor.core.publisher.Flux
-> reactor.Processors : reactor.core.publisher.Processors
-> reactor.Timers : reactor.core.timer.Timers
-> reactor.Subscribers : reactor.core.subscriber.Subscribers

This change adds a WebServerExchange and updates all contracts at the
the same level (i.e. org.springframework.web.server) as well as the
org.springframework.web.reactive level to use it so that all
framework-related code will have access to server-side processing
features such as request attributes (and others to come).

Flux and Mono are used both for implementation and exposed at API
level to express 1 versus N semantic and to provide default Rx
operators:
- Flux<T> for multiple values Publisher (issue #48)
- Mono<T> for single value Publisher (issue #50)
- Mono<Void> for Publisher with no value (issue #49)

reactive.codec -> core.codec
reactive.io    -> util

These may very well not be the final locations. For now they simply
express that there are classes that belong somewhere in core, i.e.
they are lower level dependencies than web.

The web related code is now under org.springframework.web.reactive.
This is parallel to org.springframework.web (the top-level package of
spring-webmvc).

"Content-Type" is just a single MediaType.

For the response, the MediaType must be fully selected before
selecting and encoder.

The ResponseBodyResultHandler now includes actual content negotiation
with a potential 406 response.

This commit introduces the following changes:
 - MessageToByteEncoder/Decoder renamed to Encoder/Decoder
 - JsonObjectEncoder/Decoder are now used directly in
   JacksonJsonEncoder/Decoder
 - Codec uses MimeType instead of MediaType since they
   are not specific to HTTP
 - Default MimeType are now managed thanks to
   Encoder/Decoder#getSupportedMimeTypes()
 - AbstractEncoder/Decoder takes care of generic MimeType related behavior

https://github.com/spring-projects/spring-framework/wiki/Spring-Framework-Code-Style

This commit introduces "reactive" sub-interfaces of the HttpMessage
interface found in the Spring Framework.

This commit introduces the following changes:
 - Publisher -> Observable/Stream/etc. conversion is now managed
    in a dedicated ConversionService instead of directly in
    RequestBodyArgumentResolver and ResponseBodyResultHandler
 - More isolated logic that decides if the stream should be
    serialized as a JSON array or not
 - Publisher<ByteBuffer> are now handled by regular
   ByteBufferEncoder and ByteBufferDecoder
 - Handle Publisher<Void> return value properly
 - Ensure that the headers are properly written even for response
   without body
 - Improve JsonObjectEncoder to autodetect JSON arrays

 No need for rxjava-reactive-streams dependency and
 CompletableFutureUtils anymore.

Replace Streams fail, empty and Stream map with Publishers.error, empty and map.
Replace Streams calls by Publishers.flatMap, just and Publishers.from
Precise cancel and size to toReadQueue to simulate Promise
Update build

This commit adds support for Publisher based codecs that allows to convert
byte stream to object stream and vice & versa.

Jackson, JAXB2 and String codec implementations are provided.