BUG/MEDIUM: mux-h1: Trim excess server data at the end of a transaction

BUG/MEDIUM: mux-h1: Trim excess server data at the end of a transaction

At the end of a transaction, when the conn_stream is detach from the H1
connection, on the server side, we must release the input buffer to trim any
excess data received from the server to be sure to block invalid responses.  A
typical example of such data would be from a buggy server responding to a HEAD
with some data, or sending more than the advertised content-length.

This issue was reported on Gitbub. See issue #176.

This patch must be backported to 2.0 and 1.9.