Removed in favor of accessing the factory from the response.

ViewResolutionResultHandler and ResponseBodyResultHandler now share
a common base class ContentNegotiatingResultHandlerSupport that
supports content negotiation.

For view resolution we compare against the supported media types of
resolved View instances, which may include default View's delegating
to an HttpMessageConverter (e.g. JSON, XML, rendering).

View now returns Mono<Void> rather than Flux<DataBuffer> which aligns
more closely with the reactive HttpMessageConverter vs the Encoder.

The change was prompted by the upcoming implementation of a View that
delegates to an existing HttpMessageConverter e.g. for JSON, XML.

The resulting change also brings the reactive View closer in spirit to
the View from spring-webmvc which returns void.

The ContentNegotiatingResultHandlerSupport base class encapsulates the
logic for content negotiation needed for both @ResponseBody and view
resolution result handling.

This commit adds support for Model-related return values types such as
Map, Model, @ModelAttribute annotated, and non-simple types, which
helps to clarify the logic in ViewResolutionResultHandler.

Remove handler inserted at the end to generate error in favor of doing
the same using the otherwiseIfEmpty operator.

This commit ensures correct handling for Mono.empty() return value
where the declared return type is Mono<String> or Mono<View>.

While View and ViewResolver play the same role as in spring-webmvc they
are now abstracted behind the HandlerResultHandler abstraction so that
top-level contracts don't reference them and the DispatcherHandler is
also unaware of their existence.

Furthermore view resolution and response body handling which are now at
the same level of abstraction (each is a HandlerResultHandler) will
also share code for content negotiation, so it makes sense for them to
be side by side.

This commit moves the reactive.view package to reactive.result.view
with the View and ViewResolver contracts (previously in the top-level
reactive package) also moving there.

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.

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

update after to then (WIP flux)

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").

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.