fix Message.to_dict mutating the underlying Message (#378)

* [fix] to_dict modifies the underlying message (#151)

* add test for mapmessage

* fix for to_dict

* formatting

* Apply suggestions from code review

Co-authored-by: Arun Babu Neelicattu <arun.neelicattu@gmail.com>

* change to_json to to_dict

Co-authored-by: Arun Babu Neelicattu <arun.neelicattu@gmail.com>

tests/test_mapmessage.py

@@ -0,0 +1,18 @@
+from tests.output_betterproto.mapmessage import (
+    Nested,
+    Test,
+)
+
+
+def test_mapmessage_to_dict_preserves_message():
+    message = Test(
+        items={
+            "test": Nested(
+                count=1,
+            )
+        }
+    )
+
+    message.to_dict()
+
+    assert isinstance(message.items["test"], Nested), "Wrong nested type after to_dict"