pass_chunk() was tossing a brigade composed of just the EOS bucket. on

pass_chunk() was tossing a brigade composed of just the EOS bucket. on
    entry, it saw there were zero bytes and simply returned. the client
    would never receive "end of response" and timeout (or the server would
    timeout the client).

*) added comments about what is going on
*) place chunk start/end markers around just the content, if that content
   is non-zero in length
*) insert all the right "end of response" stuff before the EOS bucket

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@86193 13f79535-47bb-0310-9956-ffa450edef68