WarningsCapture raises TypeError on warnings.warn

[d-chambers]

The WarningsCapture class in obspy.core.util.testing raises a TypeError when used with the warnings.warn function. For example:

import warnings

from obspy.core.util.testing import WarningsCapture

with WarningsCapture() as w:
    warnings.warn('something bad is happening')

This occurs because the category parameter can be none, but is not properly checked for if the message parameter is not an instance of Warning.