Is the 'raises' parameter in xfail ignored?

Hi!

My test returns xfail, although the exception type is not that I specify in 'raises'.

e. g.


```python
@pytest.mark.xfail(raises=ZeroDivisionError)
    def test_xfail(self):
        with check:
            assert 1 == 2
``` 

I am waiting 'failed', but the test returns 'xfailed'. I think I'm doing something wrong. Is there another way?
