BUG/MINOR: http: Establish a tunnel for all 2xx responses to a CONNECT

BUG/MINOR: http: Establish a tunnel for all 2xx responses to a CONNECT

As stated in the rfc7231, section 4.3.6, an HTTP tunnel via a CONNECT method
is successfully established if the server replies with any 2xx status
code. However, only 200 responses are considered as valid. With this patch,
any 2xx responses are now considered to estalish the tunnel.

This patch may be backported on demand to all stable versions and adapted
for the legacy HTTP. It works this way since a very long time and nobody
complains.