mod_cache: Wrong content type and character set when mod_cache serves

stale content because of a proxy error. PR 53539. Correction to r1361153.
trunk patch: http://svn.apache.org/viewvc?view=revision&revision=1373447
2.4.x patch: trunk patch works.
+1: rjung, jim, minfrin


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1393191 13f79535-47bb-0310-9956-ffa450edef68