Handle mutable default arguments cleanly

When generating code, ensure that default list/dict arguments are
initialised in local scope if unspecified or `None`.

tests/grpc/test_grpclib_client.py

@@ -1,11 +1,14 @@
 import asyncio
+import sys
+
 from tests.output_betterproto.service.service import (
-    DoThingResponse,
     DoThingRequest,
+    DoThingResponse,
     GetThingRequest,
     TestStub as ThingServiceClient,
 )
 import grpclib
+import grpclib.metadata
 from grpclib.testing import ChannelFor
 import pytest
 from betterproto.grpc.util.async_channel import AsyncChannel
@@ -35,6 +38,20 @@ async def test_simple_service_call():
         await _test_client(ThingServiceClient(channel))
 
 
+@pytest.mark.asyncio
+@pytest.mark.skipif(
+    sys.version_info < (3, 8), reason="async mock spy does works for python3.8+"
+)
+async def test_service_call_mutable_defaults(mocker):
+    async with ChannelFor([ThingService()]) as channel:
+        client = ThingServiceClient(channel)
+        spy = mocker.spy(client, "_unary_unary")
+        await _test_client(client)
+        comments = spy.call_args_list[-1].args[1].comments
+        await _test_client(client)
+        assert spy.call_args_list[-1].args[1].comments is not comments
+
+
 @pytest.mark.asyncio
 async def test_service_call_with_upfront_request_params():
     # Setting deadline

tests/inputs/service/service.proto

@@ -4,6 +4,7 @@ package service;
 
 message DoThingRequest {
   string name = 1;
+  repeated string comments = 2;
 }
 
 message DoThingResponse {