'Assert that the class name of a L{mail.mail.DomainWithDefaultDict}
instance and the string-formatted underlying domain dictionary both
appear in the string produced by the given string-returning function.
@type stringifier: one-argument callable
@param stringifier: either C{str} or C{repr}, to be used to get a
string to make assertions against.'

def _stringificationTest(self, stringifier):

    domain = mail.mail.DomainWithDefaultDict({}, 'Default')
    self.assertIn(domain.__class__.__name__, stringifier(domain))
    domain['key'] = 'value'
    self.assertIn(str({'key': 'value'}), stringifier(domain))
