Merge r1153531 from trunk:

* Do not even sent an empty brigade down the filter chain if the headers
  have not been sent by the AJP server so far. Even an empty brigade
  will trigger the headers filter to create the (in this case incomplete)
  HTTP headers of the response.

PR: 51608
Submitted by: rpluem
Reviewed by: rpluem, jim, jfclere


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1166611 13f79535-47bb-0310-9956-ffa450edef68