This commit adds support for handling an empty request body with both
HttpEntity where the body is not required and with @RequestBody where
the body is required depending on the annotation's required flag.

If the body is an explicit type (e.g. String, HttpEntity<String>) and
the body is required an exception is raised before the method is even
invoked or otherwise the body is passed in as null.

If the body is declared as an async type (e.g. Mono<String>,
HttpEntity<Mono<String>>) and is required, the error will flow through
the async type. If not required, the async type will be passed with no
values (i.e. empty).

A notable exception is rx.Single which can only have one value or one
error and cannot be empty. As a result currently the use of rx.Single
to represent the request body in any form effectively implies the body
is required.