BUG/MINOR: htpp-ana/stats: Specify that HTX redirect messages have a C-L header

BUG/MINOR: htpp-ana/stats: Specify that HTX redirect messages have a C-L header

Redirect responses sent during the HTTP analysis have no payload. However
there is still a "Content-Length" header. It is important to set the
corresponding flag on the HTX start-line to be sure to preserve this header
when the reponse is sent to the client. The same is true with the stats
applet, when it returns a redirect responses.

It is especially important because we no ignore in-fly modifications of
"Content-Length" or "Transfer-Encoding" headers without updating the HTX
start-line flags.

This patch may be backported to all stable versions but it is probably
useless because only the 2.9-dev is affected by the bug.