backport r791454 from 2.2.x branch:

SECURITY: CVE-2009-1891 (cve.mitre.org)
Fix a potential Denial-of-Service attack against mod_deflate or other 
modules, by forcing the server to consume CPU time in compressing a 
large file after a client disconnects.  [Joe Orton, Ruediger Pluem]

Submitted by: jorton, rpluem
Reviewed by: pgollucci, poirier, rjung


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.0.x@1001425 13f79535-47bb-0310-9956-ffa450edef68