Add upgrade parameter to mod_proxy_wstunnel.

That allows to upgrade to jboss-remoting for example
or to run an HTTP/1.1 backend that needs to upgrade to
WebSocket.
See also:
 https://issues.jboss.org/browse/JBCS-254
 https://issues.jboss.org/browse/JBCS-291


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1792092 13f79535-47bb-0310-9956-ffa450edef68