Revision 425725

Merge r410370 from trunk:

* Keep the Content-Type for successfully revalidated
  cached objects, by unsetting possible Content-Type headers in
  r->headers_out and r->err_headers_out as they may be different to what
  we have received from the cache.
  Actually they are not needed as r->content_type set by
  ap_set_content_type a few lines above will be used in the store_headers
  functions of the storage providers as a fallback and the HTTP_HEADER filter
  does overwrite the Content-Type header with r->content_type anyway.

PR: 39647
Submitted by: rpluem
Reviewed by: rpluem, pquerna, fielding