BUG/MEDIUM: mux-h1: Close client H1C on EOS when there is no output data

BUG/MEDIUM: mux-h1: Close client H1C on EOS when there is no output data

If the client closes the connection while there is no pending outgoing data,
the H1 connection must be released. However, it was switched to CLOSING
state instead. Thus the client connection was closed on client timeout.

It is side effect of the commif d1b573059a ("MINOR: mux-h1: Avoid useless
call to h1_send() if no error is sent"). Before, the extra call to h1_send()
was able to fix the H1C state.

To fix the bug and make switch to close state (CLOSING or CLOSED) less
errorprone, h1_close() helper function is systematically used.

It is a 2.7-specific bug. No backport needed.