Backport of r357328 from trunk:

* Fix ap_http_chunk_filter to create correct chunks in the case that a
  flush bucket is pre- and postfixed by data buckets in the brigade.

Submitted by: Ruediger Pluem
Reviewed by: Justin Erenkrantz, Joe Orton, Jim Jagielski


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.0.x@357454 13f79535-47bb-0310-9956-ffa450edef68