svnpubsub: fix MIME type declaration.  Reincarnation of r1486775.

application/json is wrong: it is not appropriate for a sequence of
NUL-terminated JSON objects.

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

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1486809 13f79535-47bb-0310-9956-ffa450edef68