PATCH without JSON raises a 500 error.

[Natim]

File "cliquet/resource.py", line 398, in patch
    changes = self.request.json.get('data', {})  # May patch only perms.
  File "webob/request.py", line 725, in _json_body__get
    return json.loads(self.body.decode(self.charset))
ValueError: Expected object or value

[leplatrem]

This was supposed to be tested here : https://github.com/mozilla-services/cliquet/blob/master/cliquet/tests/resource/test_views.py#L327-L332