BUG/MEDIUM: mux-h1: Handle TUNNEL state when outgoing messages are formatted

BUG/MEDIUM: mux-h1: Handle TUNNEL state when outgoing messages are formatted

Since the commit 94b2c7 ("MEDIUM: mux-h1: refactor output processing"), the
formatting of outgoing messages is performed on the message state and no more on
the HTX blocks read. But the TUNNEL state was left out. So, the HTTP tunneling
using the CONNECT method or switching the protocol (for instance,
the WebSocket) does not work.

This issue was reported on Github. See #131. This patch must be backported to
2.0.