Comment 2 for bug 1533270

Co-incidently, this is what I was hitting when testing https://review.openstack.org/#/c/254873

  File "/usr/local/lib/python2.7/dist-packages/webob/dec.py", line 195, in call_func
    return self.func(req, *args, **kwargs)
  File "/mnt/opt/stack/glance/glance/common/wsgi.py", line 588, in __call__
    response = req.get_response(self.application)
  File "/usr/local/lib/python2.7/dist-packages/webob/request.py", line 1317, in send
    application, catch_exc_info=False)
  File "/usr/local/lib/python2.7/dist-packages/webob/request.py", line 1281, in call_application
    app_iter = application(self.environ, start_response)
  File "/usr/local/lib/python2.7/dist-packages/webob/dec.py", line 130, in __call__
    resp = self.call_func(req, *args, **self.kwargs)
  File "/usr/local/lib/python2.7/dist-packages/webob/dec.py", line 195, in call_func
    return self.func(req, *args, **kwargs)
  File "/mnt/opt/stack/glance/glance/common/wsgi.py", line 585, in __call__
    response = self.process_request(req)
  File "/mnt/opt/stack/glance/glance/api/middleware/cache.py", line 171, in process_request
    return method(request, image_id, image_iterator, image_metadata)
  File "/mnt/opt/stack/glance/glance/api/middleware/cache.py", line 226, in _process_v2_request
    self._verify_metadata(image_meta)
  File "/mnt/opt/stack/glance/glance/api/middleware/cache.py", line 75, in _verify_metadata
    image_meta['size'] = self.cache.get_image_size(image_meta['id'])
 TypeError: 'ImageTarget' object does not support item assignment