Merge r1718569 from trunk.

h2_from_h1: To avoid any further confusion, r->err_headers should be cleared
once merged into r->headers_out.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1718586 13f79535-47bb-0310-9956-ffa450edef68