XhrPolling.test_transport fails because of the bug at verify_content_type

Source code of verify_content_type:

```
def verify_content_type(self, r, content_type):
    self.assertEqual(r['content-type'].replace(' ', ''), content_type)
```

It always fails when content_type contains space:

```
self.verify_content_type(r, 'text/plain; charset=UTF-8')
```
