In the content-length+bytes-sent filter, only set the content length

if we decided to do so initially *and* we've seen all the data.

Prior to this change, for a http/1.1 request to mod_autoindex, the
unconditional call to ap_set_content_length() gave us the wrong
value (only taking into account the first brigade) and disabled
chunking.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@86874 13f79535-47bb-0310-9956-ffa450edef68