BUG/MEDIUM: mux-h1: Continue to process request when switching in tunnel mode

BUG/MEDIUM: mux-h1: Continue to process request when switching in tunnel mode

When input data are processed, if the request is switched in tunnel mode on a
protocol upgrade, we must continue the processing. Otherwise, pending input data
will only be processed on the next wakeup. So when new input data are received,
on a timeout expiration or shutdown. Worst, if the input buffer is full when it
happens, only a timeout or a shutdown will unblock the situation.

This patch should fix the issue #737. It must be backported as far as 1.9. The
bug does not seem to affect the 2.0 and 1.9 because, on a protocol upgrade, the
request is switched in tunnel mode when the response is sent to the client. But
the bug is present, so the backport remains necessary.