BUG/MEDIUM: http: Switch the HTTP response in tunnel mode as earlier as possible

BUG/MEDIUM: http: Switch the HTTP response in tunnel mode as earlier as possible

When the body length is undefined (no Content-Length or Transfer-Encoding
headers), The reponse remains in ending mode, waiting the request is done. So,
most of time this is not a problem because the resquest is done before the
response. But when a client sends data to a server that replies without waiting
all the data, it is really not desirable to wait the end of the request to
finish the response.

This bug was introduced when the tunneling of the request and the reponse was
refactored, in commit 4be980391 ("MINOR: http: Switch requests/responses in
TUNNEL mode only by checking txn flag").

This patch should be backported in 1.8 and 1.7.