BUG/MEDIUM: mux-h2: do not abort HEADERS frame before decoding them

BUG/MEDIUM: mux-h2: do not abort HEADERS frame before decoding them

If a response HEADERS frame arrives on a closed connection (due to a
client abort sending an RST_STREAM), it's currently immediately rejected
with an RST_STREAM, like any other frame. This is incorrect, as HEADERS
frames must first be decoded to keep the HPACK decoder synchronized,
possibly breaking subsequent responses.

This patch excludes HEADERS/CONTINUATION/PUSH_PROMISE frames from the
central closed state test and leaves to the respective frame parsers
the responsibility to decode the frame then send RST_STREAM.

This fix must be backported to 1.9. 1.8 is not directly impacted since
it doesn't have response HEADERS nor trailers thus cannot recover from
such situations anyway.