[V3 Config] Fix for unnecessary writes on context mgr exit (#1859)

* Fix for #1857

* And copy it so we don't have mutability issues

* Add a test

tests/core/test_config.py

@@ -1,3 +1,4 @@
+from unittest.mock import patch
 import pytest
 
 
@@ -399,6 +400,16 @@ async def test_ctxmgr_no_shared_default(config, member_factory):
     assert 1 not in await config.member(m2).foo()
 
 
+@pytest.mark.asyncio
+async def test_ctxmgr_no_unnecessary_write(config):
+    config.register_global(foo=[])
+    foo_value_obj = config.foo
+    with patch.object(foo_value_obj, "set") as set_method:
+        async with foo_value_obj() as foo:
+            pass
+        set_method.assert_not_called()
+
+
 @pytest.mark.asyncio
 async def test_get_then_mutate(config):
     """Tests that mutating an object after getting it as a value doesn't mutate the data store."""