BUG/MAJOR: mux-h2: Handle HEADERS frames received after a RST_STREAM frame

BUG/MAJOR: mux-h2: Handle HEADERS frames received after a RST_STREAM frame

As stated in the RFC7540#5.1, an endpoint that receives any frame other than
PRIORITY after receiving a RST_STREAM MUST treat that as a stream error of type
STREAM_CLOSED. However, frames carrying compression state must still be
processed before being dropped to keep the HPACK decoder synchronized. This had
to be the purpose of the commit 8d9ac3ed8b ("BUG/MEDIUM: mux-h2: do not abort
HEADERS frame before decoding them"). But, the test on the frame type was
inverted.

This bug is major because desynchronizing the HPACK decoder leads to mixup
indexed headers in messages. From the time an HEADERS frame is received and
ignored for a closed stream, wrong headers may be sent to the following streams.

This patch may fix several bugs reported on github (#116, #290, #292). It must
be backported to 2.0 and 1.9.