'Test for #30.
If you explicitly declare ChoiceFilter fields you **MUST** pass `choices`.'

def test_filtering_on_explicitly_defined_field(self):

    class F(FilterSet, ):
        status = ChoiceFilter(choices=STATUS_CHOICES)
        class Meta:
            model = User
            fields = [u'status']
    f = F()
    self.assertQuerysetEqual(f.qs, [u'aaron', u'alex', u'jacob', u'carl'], (lambda o: o.username), False)
    f = F({u'status': u'1'})
    self.assertQuerysetEqual(f.qs, [u'alex'], (lambda o: o.username), False)
    f = F({u'status': u'2'})
    self.assertQuerysetEqual(f.qs, [u'jacob', u'aaron'], (lambda o: o.username), False)
    f = F({u'status': u'0'})
    self.assertQuerysetEqual(f.qs, [u'carl'], (lambda o: o.username), False)
