Make the header filter use the brigade buffering functions for creating

the header string.  This allows us to clean up the header handling a bit,
because we don't need to compute the correct length before we can create
the headers.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@88029 13f79535-47bb-0310-9956-ffa450edef68