Merge of r1749505, r1749676 from trunk:

mod_proxy_http2: fixed retry behaviour when proxy engine needs to retry its base request on a new connection



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1749684 13f79535-47bb-0310-9956-ffa450edef68