Before this commit, a handler method returning a stream with a JSON
content-type was producing a JSON object for single element streams
or a JSON array for multiple elements streams.
This kind of dynamic change of the output based on the number of
elements was difficult to handle on client side and not consistent
with Spring MVC behavior.
With this commit, we achieve a more consistent behavior by using
the Mono semantics to control this behavior. Mono (and Promise/Single)
are serialized to JSON object and Flux (and Observable/Stream) are
serialized to JSON array.
d9b67f5e72