Merge of r1747735 from trunk:

mod_proxy_http2: properly care for HTTP2 flow control of the frontend
     connection is HTTP/1.1. [Patch supplied by Evgeny Kotkov]



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1748168 13f79535-47bb-0310-9956-ffa450edef68