clean up the data that we just wrote to the spool file;

otherwise, very very nasty stuff happens when input brigade
with same contents is passed down to filters to grab more
body data


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/proxy-reqbody@124204 13f79535-47bb-0310-9956-ffa450edef68