When a null is returned from an @ResponseBody method, rather than
returning Mono.empty() immediately, convert it to Mono.empty() and
apply the same processing.

Currently that doesn't make a practical difference but it's more
accurate to do it this way. Eventually it may mean the possibility
to turn empty values into something through an extension point
as we do with ResponseBodyAdvice in Spring MVC today.

Issue #103

 - @PathVariable
 - @RequestHeader
 - @RequestParam
 - @CookieValue
 - @Value
 - @RequestAttribute
 - @SessionAttribute

Replaced all usages of toInputStream with non-blocking alternatives.

update after to then (WIP flux)

This commit introduces a pooled data buffer as a subtype of DataBuffer,
as well as various utility methods related to reference counting.

Additionally, Crelease calls have been introduced throughout the
codebase to properly dispose of pooled databuffers.

Use CharsetDecoder to decode a DataBuffer into a String.

This commit introduces the AbstractSingleValueEncoder, an abstract base
class for encoders that only handle a single value.

This commit introduces the ResourceEncoder and ResourceDecoder, and uses
these in ResourceHttpMessageConverter as a non-zero-copy fallback
method.

Rename two classes each adapting to WebHandler to avoid confusing them:
1. HttpWebHandlerAdapter adapts from the low level HttpHandler to any
WebHandler (e.g. DispatcherHandler).
2. SimpleHandlerAdapter adapts the plain WebHandler for use within the
DispatcherHandler.

This commit also fixes an issue in WebHttpHandlerBuilder to ensure that
WebExceptionHandler's are inserted before and not after WebFilter's.

This commit replaces the temporary SimpleUrlHandlerMapping with
a full-featured one.

The renaming makes it clear this exception is for use where 400 error
applies within a Spring web application where the error may be
associated with a MethodParameter, a BindingResult, and so on.

There is no need for BadRequestStatusException which can be expressed
with ResponseStatusException(HttpStatus.BAD_REQUEST, "reason").

@ResponseBody Resource support

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.

This commit introduces a reactive version of the HttpMessageConverter.
During the implementation of zero-copy support, it became apparent that
it was ueful to have a common abstraction between client and server that
operated on HttpMessages rather than DataBuffers.

Two HttpMessageConverter implementations are provided:
 - The CodecHttpMessageConverter, based on Encoder/Decoder.
 - The ResourceHttpMessageConverter, using zero-copy if available.

This commit introduces support for zero-copy file transfers in the HTTP
response, through the ZeroCopyHttpOutputMessage subinterface of
ReactiveHttpOutputMessage.

This commit introduces two DataBuffer improvements:

 - The capability to read a Flux<DataBuffer> from an input stream or
   channel.

 - The capability to limit a Publisher<DataBuffer> to publish up until a
   given maximum byte count.