Add tests for Mapping key type-hooks

tests/core/test_config.py

@@ -1,6 +1,6 @@
 from dataclasses import dataclass
 from enum import Enum
-from typing import Optional, List, Union
+from typing import Optional, List, Union, Dict
 
 import pytest
 
@@ -121,6 +121,16 @@ def raise_error(_):
         from_dict(X, {"i": 1}, config=Config(type_hooks={int: raise_error}))
 
 
+def test_from_dict_with_type_hooks_and_generic_mapping():
+    @dataclass
+    class X:
+        d: Dict[str, str]
+
+    result = from_dict(X, {"d": {"B": "TEST"}}, config=Config(type_hooks={str: str.lower}))
+
+    assert result == X(d={"b": "test"})
+
+
 def test_from_dict_with_forward_reference():
     @dataclass
     class X: