Do not send a content-length if and only if this is a HEAD request and

Do not send a content-length if and only if this is a HEAD request and
the content-length is 0.  The problem is that the C-L on a HEAD response
has to be the correct C-L, but if a handler returns saying the handled
the request without sending data, the core sends an EOS down the filter
stack, and we compute a 0 C-L.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@86976 13f79535-47bb-0310-9956-ffa450edef68