Merge of r1769596,1769600,1770395,1770998 from trunk

mod_http2: PUSH triggers only on GET
mod_proxy_http2: 1xx responses not forwarded unconditionally on HTTP/1.x connections


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1771000 13f79535-47bb-0310-9956-ffa450edef68