Before this change the write Publisher was saved and Mono.empty()
returned from the write metohd which did not properly implement
the write contract since no writing ("consuming") was done.

This can be a problem in some cases. For example the request may appear
to succeed even if the publisher produces an error later when
subscribed to later after request handling completes.

This commit introduces a writeHandler function in the mock request and
response. By default it "writes" by consuming the content immediately,
which allows it to return a Mono<Void> that properly reflects when
writing is done, and it also caches the data so it may be replayed
later for test assertions.

For streaming scenario a custom writeHandler may be registered which
allows the custom handling to determine how long to stream before
cancelling so request handling may complete.

Issue: SPR-14590