[HttpServer] Outbound send shouldn't subscribe if connection is disposed

[smaldini]

since #741 has been fixed, we need to revisit how to prevent subscribing to passed publisher for prematurely cancelled flows: The fix is deferring the cancel therefore the Publisher.subscribe can be called.

We should be able to add a predicate to MonoSend to do a late check (in that case we shouldn't call subscribe if body has been sent already).