core: Add the ability to do explicit matching on weak and strong ETags

as per RFC2616 Section 13.3.3.

trunk patch: http://svn.apache.org/r1479528

Submitted by: minfrin
Reviewed by: jim, wrowe


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1487123 13f79535-47bb-0310-9956-ffa450edef68