BUG/MINOR: mux-h2: make sure response HEADERS are not received in other states than...

BUG/MINOR: mux-h2: make sure response HEADERS are not received in other states than OPEN and HLOC

RFC7540#5.1 states that these are the only states allowing any frame
type. For response HEADERS frames, we cannot accept that they are
delivered on idle streams of course, so we're left with these two
states only. It is important to test this so that we can remove the
generic CLOSE_STREAM test for such frames in the main loop.

This must be backported to 1.9 (1.8 doesn't have response HEADERS).