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

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

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 replaces use of the existing ServletException-based
exceptions with the new ones from ~.web.server.

Now that we have a CompositeContentTypeResolverBuilder with built-in
defaults, we switch to those defaults in places where a
ContentTypeResolver is used.

Rename to MappingContentTypeResolver and replace "fileExtension" with
"key" in methods to be more general (e.g. file extension or query
param). Also switch from List to Set for the return values.

This commit adds RequestMappingInfoHandlerMapping and
RequestMappingHandlerMapping with support equivalent to that in
spring-webmvc.

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