Set serialized_on_wire when parsing

This is similar to the fix at danielgtaylor#81, but that doesn't merge into our branch because our branch is too old. So this just pulls in the relevant fix part.

betterproto/__init__.py

@@ -664,6 +664,7 @@ def parse(self: T, data: bytes) -> T:
         fields = {f.metadata["betterproto"].number: f for f in dataclasses.fields(self)}
         for parsed in parse_fields(data):
             if parsed.number in fields:
+                self._serialized_on_wire = True
                 field = fields[parsed.number]
                 meta = FieldMetadata.get(field)