Revision 1885605

Backport to v2.4:

  *) mod_proxy_http: handle upgrade/tunneling protocols. BZ 61616 is about
                     mod_proxy_connect but there has been wstunnel reports
                     on dev@ about that too lately.
     trunk patch: https://svn.apache.org/r1678771
                  https://svn.apache.org/r1832348
                  https://svn.apache.org/r1869338
                  https://svn.apache.org/r1869420
                  https://svn.apache.org/r1878367
                  https://svn.apache.org/r1877557
                  https://svn.apache.org/r1877558
                  https://svn.apache.org/r1877646
                  https://svn.apache.org/r1877695
                  https://svn.apache.org/r1879401
                  https://svn.apache.org/r1879402
                  https://svn.apache.org/r1880200
                  https://svn.apache.org/r1885239
                  https://svn.apache.org/r1885240
                  https://svn.apache.org/r1885244
     2.4.x patch: http://people.apache.org/~ylavic/patches/2.4.x-mod_proxy_http-upgrade-4on5-v2.patch
                  https://github.com/apache/httpd/pull/158
     +1: ylavic, covener, minfrin
     ylavic: All the corresponding trunk changes to mod_proxy_wstunnel (but
             r1885239) have been dropped for this backport proposal, the goal
             being to handle upgrade in mod_proxy_http from now, while r1885239
             allows to benefit from the Upgrade improvements done in proxy_http
             with existing wstunnel configurations (provided mod_proxy_http
             module is loaded).