Change the content-length filter to give up (i.e., not provide content-length)

Change the content-length filter to give up (i.e., not provide content-length)
if it gets a flush bucket.  A generator of streaming content needs to send down
a flush bucket (e.g., by calling ap_rflush()) if it wants to send a partial
response to the client immediately.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@86826 13f79535-47bb-0310-9956-ffa450edef68