Merge r607466, r607472, r607838 from trunk:

mod_dav: Fix evaluation of If-Match * and If-None-Match * conditionals.
PR 38034
Patch by Paritosh Shah
Explanation by Werner Baumann

* Style police. No functional changes.

* Avoid SEGFAULT as hooks->set_headers can be NULL. Furthermore do not set the
  ETag header permanently as setting it may not be desired for all responses.

Submitted by: niq, rpluem
Reviewed by: niq, rpluem, chrisd


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@609113 13f79535-47bb-0310-9956-ffa450edef68