etag attribute on an iterator doesn't change

Test case:

``` python
g = github3.login('username', 'password')
i = g.iter_gists()
gists = list(i)
print(i.etag)
g.create_gist('Test', {'test': {'content': 'test'}}) 
i.refresh(True)
new_gists = list(i)
print(i.last_response.headers['ETag'])
print(i.etag)
```

The value of the `ETag` header returned is different from the attribute because the attribute retained its old value.
