Update dict copy tests

Author: vsakkas

mypyc/test-data/run-dicts.test

@@ -92,14 +92,17 @@ assert get_content(od) == ([3, 1], [4, 2], [(3, 4), (1, 2)])
 assert get_content_set({1: 2}) == ({1}, {2}, {(1, 2)})
 assert get_content_set(od) == ({1, 3}, {2, 4}, {(1, 2), (3, 4)})
 
+from typing import Dict
 from collections import defaultdict
 d = {}
 assert d.copy() == d
 d = {'a': 1, 'b': 2}
 assert d.copy() == d
 assert d.copy() is not d
-d = defaultdict(int)
-assert d.copy() == d
+dd: Dict[str, int] = defaultdict(int)
+dd['a'] = 1
+assert dd.copy() == dd
+assert isinstance(dd.copy(), defaultdict)
 [typing fixtures/typing-full.pyi]
 
 [case testDictIterationMethodsRun]