[mod_proxy_]http: follow up to r1750392.

Export [ap_]check_pipeline() and use it also for ap_proxy_check_connection(),
so that all the necessary checks on the connection are done before reusing it.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1756186 13f79535-47bb-0310-9956-ffa450edef68