Merge r1610814, r1610686, r1610707 from trunk:

Merge r1610814, r1610686, r1610707 from trunk:

      *) SECURITY: CVE-2013-5704 (cve.mitre.org)
         core: HTTP trailers could be used to replace HTTP headers
         late during request processing, potentially undoing or
         otherwise confusing modules that examined or modified
         request headers earlier.  Adds "MergeTrailers" directive to restore
         legacy behavior.

    Submitted By: Edward Lu, Yann Ylavic, Joe Orton, Eric Covener
    Committed By: covener

Reviewed By:  covener, wrowe, rpluem

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1619489 13f79535-47bb-0310-9956-ffa450edef68