If user sends "Accept: */*" we don't want to default to sending
an SSE (for instance). So the logic for detecting those preferences
has to take MediaType.ALL into account as a special case.
We don't need to cover all the possible uses of Flux (only
Flux<String> really), so this isn't comprehensive coverage of
all the features in Spring WebFlux, but it's good enough for
functions to run with Spring Boot 1.5.
User can POST to web endpoint in SSE style, i.e:
HTTP/1.1
Content-Type: text/event-stream
data:foo
data:bar
Will be converted to a Flux with values foo and bar
