BUG/MINOR: mux-h2: Don't encroach on the reserve when decoding headers

BUG/MINOR: mux-h2: Don't encroach on the reserve when decoding headers

Since the input buffer is transferred to the stream when it is created,
there is no longer control on the request size to be sure the buffer's
reserve is still respected. It was automatically performed in h2_rcv_buf()
because the caller took care to provide the correct available space in the
buffer. The control is still there but it is no longer applied on the
request headers. Now, we should take care of the reserve when the headers
are decoded, before the stream creation.

The test is performed for the request and the response.

It is a 2.4-specific bug. No backport is needed.