svnpubsub: fix MIME type declaration.

text/plain is wrong due to embedded NULs.

[in tools/server-side/svnpubsub]
* svnpubsub/server.py
  (SvnPubSub.render_GET): Use application/octet-stream.  Leave a comment about
    exploring better options.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1486775 13f79535-47bb-0310-9956-ffa450edef68