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.

Added DataBufferAllocator property to ReactiveHttpOutputMessage, and use that to pass on to Encoder.encode().

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.

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).

This change allows to be able to check generic type on the return value
at HandlerAdapter and ResultHandler level. For example, it allows to do
a Publisher<Void> check in SimpleHandlerResultHandler.

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

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

This commit updates HandlerAdapter#handle() to return HandlerResult
instead of Publisher<HandlerResult>. A new SimpleHandlerResultHandler
class has been introduced for handlers returning Publisher<Void>.