Instead of accepting List<Encoder|Decoder> and then look for the first
to support JSON, always expect a single JSON [Encoder|Decoder] and use
that unconditionally.
When writing use the nested ResolvableType instead of the Class of the
actual value which should better support generics.
Remove the SSE hint and pass "text/event-stream" as the media type
instead to serve as a hint. We are expecting a JSON encoder and using
it unconditionally in any case so this should be good enough.