Fix: page data not being returned when using PyQt

This solves the issue mentioned at the bottom of #53, that page returns empty:

```
>>> from ghost import Ghost
>>> ghost = Ghost()
>>> page, extra_resources = ghost.open("http://jeanphi.fr")
>>> assert page.http_status==200 and 'jeanphix' in ghost.content
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'http_status'
```

This is due to the use of `.toString()` on the `url` variable when using PyQt, while comparing with a `QUrl` object.