* mod_headers: Allow for format specifiers to be used in the base

      substitution string when using Header edit
          trunk patch: http://people.apache.org/~druggeri/patches/HeaderEditENV.trunk.patch
              2.4.x patch: http://people.apache.org/~druggeri/patches/HeaderEditENV.2.4.patch
                  +1: druggeri, covener, jim



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1540313 13f79535-47bb-0310-9956-ffa450edef68