Fix issue where dynamic property isn't on request

In some cases the middleware returns a response before all of the
middlewares have been gone through. When this happens a dynamic
attribute is not attached to the request object.

This PR just returns the response if the attribute is not there
since that means there aren't any expiriments.
How about this instead:
```python
experiments = getattr(request, 'variant_experiments', {})

for name, variant in six.iteritems(experiments):
    ....
```