http: Don't remove the Content-Length of zero from a HEAD response if

it comes from an origin server, module or script.  

Instead of removing the C-L in ap_http_header_filter() after it has been
set by ap_content_length_filter(), just don't set it in the latter.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1678215 13f79535-47bb-0310-9956-ffa450edef68