mod_deflate buffers responses

mod_deflate buffers entire responses, which blocks any streamy
responses coming from mod_dav_svn.

This is technically an Apache bug, but placing into svn's issuetracker
as a constant reminder to kick the apache team into fixing it.  It
creates huge scalability problems for Subversion.

(In svn 0.26 and later, neon does *not* accept-gzip encodings by
default.  That's our temporary workaround.)