Proxy: support for connection upgrade (101 Switching Protocols).

This allows to proxy WebSockets by using configuration like this:

    location /chat/ {
        proxy_pass http://backend;
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "upgrade";
    }

Connection upgrade is allowed as long as it was requested by a client
via the Upgrade request header.


git-svn-id: svn://svn.nginx.org/nginx/trunk@5073 73f98a42-aea0-e011-b76d-00259023448c