Before this commit only ResponseEntity with async body was supported,
e.g. ResponseEntity<Mono<String>>

This commit also adds suppport for an asyn wrapper around,
e.g. Mono<ResponseEntity<String>.

Introduce separate test classes for each base class in the hierarchy
above @ResponseBody and ResponseEntity result handlers.

Also start porting existing unit test cases for @ResponseBody and
ResponseEntity return value handlers.

CodecHttpMessageConverter is now suitable for SSE since it now
handles default content type.

Reactored Servlet 3.1 and Undertow response support into an
AbstractResponseBodySubscriber that uses an internal state machine,
making thread-safity a lot easier.

Servlet flush will now occur on the next possible write if it cannot be
done immediately.

Flux<SseEvent> is Spring Web Reactive equivalent to Spring MVC
SseEmitter type. It allows to send Server-Sent Events in a reactive way.
Sending Flux<String> or Flux<Pojo> is equivalent to sending
Flux<SseEvent> with the data property set to the String or
Pojo value. For example:

@RestController
public class SseController {

	@RequestMapping("/sse/string")
	Flux<String> string() {
		return Flux.interval(Duration.ofSeconds(1)).map(l -> "foo " + l);
	}

	@RequestMapping("/sse/person")
	Flux<Person> person() {
		return Flux.interval(Duration.ofSeconds(1)).map(l -> new Person(Long.toString(l), "foo", "bar"));
	}

	@RequestMapping("/sse-raw")
	Flux<SseEvent> sse() {
		return Flux.interval(Duration.ofSeconds(1)).map(l -> {
			SseEvent event = new SseEvent();
			event.setId(Long.toString(l));
			event.setData("foo\nbar");
			event.setComment("bar\nbaz");
			return event;
		});
	}
}

This commit add flushing support thanks to the FlushingDataBuffer
wrapper that allows to identify the elements that should trigger a
flush.

This commit also fixes an issue in the HTTP client that used the
wrapper type instead of the element type. As a consequence, due
to type erasure, we now have to specify the type of the content
in DefaultHttpRequestBuilder#contentStream().

- Added fromIndex parameter to indexOf and lastIndexOf
- Moved DataBuffer.tokenize to StringEncoder, as that's the only place
  it's used.

Renamed getSupportedMimeTypes() to getEncodableMimeTypes and
getDecodableMimeTypes. This will allow for both Encoder and Decoder to
be implemented in the same class.

This issue fixes #113.

The Pojo test class from the codec package will end up in spring-core.
This commit ensures it is used only from classes that also belong to
spring-core.

Removed in favor of accessing the factory from the response.