get_by_id should be aware of previous filters

[kiddouk]

Considering this case :

class A(models.Model):
    user = models.ReferenceField(B)

class B(models.Model):
    att = models.Attribute()


b1 = B(att='test)
b1.save()

b2 = B(att='test')
b2.save()

a = A(user=b1)
a.save()

self.assertEqual(a.user_set.get_by_id(b1.id), b1)
self.assertEqual(a.user_set.get_by_id(b2.id), None)

The second test will fail as get_by_id doesn't take into account any filter/zfilter/excludes.