Added switch_collection context manager and method (#220)

tests/test_connection.py

@@ -95,30 +95,6 @@ class ConnectionTest(unittest.TestCase):
         date_doc = DateDoc.objects.first()
         self.assertEqual(d, date_doc.the_date)
 
-    def test_switch_db_context_manager(self):
-        connect('mongoenginetest')
-        register_connection('testdb-1', 'mongoenginetest2')
-
-        class Group(Document):
-            name = StringField()
-
-        Group.drop_collection()
-
-        Group(name="hello - default").save()
-        self.assertEqual(1, Group.objects.count())
-
-        with switch_db(Group, 'testdb-1') as Group:
-
-            self.assertEqual(0, Group.objects.count())
-
-            Group(name="hello").save()
-
-            self.assertEqual(1, Group.objects.count())
-
-            Group.drop_collection()
-            self.assertEqual(0, Group.objects.count())
-
-        self.assertEqual(1, Group.objects.count())
 
 if __name__ == '__main__':
     unittest.main()