MINOR: mux-h2: Set H2_SF_ES_RCVD flag when decoding the HEADERS frame

MINOR: mux-h2: Set H2_SF_ES_RCVD flag when decoding the HEADERS frame

The flag H2_SF_ES_RCVD is set on the H2 stream when the ES flag is found in
a frame. On HEADERS frame, it was set in function processing the frame. It
is moved in the function decoding the frame. Fundamentally, this changes
nothing. But it will be useful to have this information earlier when a
client H2 stream is created.