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

src/betterproto/__init__.py

@@ -1124,12 +1124,13 @@ def to_dict(
                 ):
                     output[cased_name] = value.to_dict(casing, include_default_values)
             elif meta.proto_type == TYPE_MAP:
+                output_map = {**value}
                 for k in value:
                     if hasattr(value[k], "to_dict"):
-                        value[k] = value[k].to_dict(casing, include_default_values)
+                        output_map[k] = value[k].to_dict(casing, include_default_values)
 
                 if value or include_default_values:
-                    output[cased_name] = value
+                    output[cased_name] = output_map
             elif (
                 value != self._get_field_default(field_name)
                 or include_default_values

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"