model: Implement DmRecipientListConverter toJson method.

DmRecipientListConverter toJson was unimplemented, and just returned the list of DMRecipient objects. This changes it to return JSON representing the DMRecipients, which allows a DMMessage to be serialized and deserialized.

lib/api/model/model.dart

@@ -383,7 +383,7 @@ class DmRecipientListConverter extends JsonConverter<List<DmRecipient>, List<dyn
   }
 
   @override
-  List toJson(List<DmRecipient> object) => object;
+  List toJson(List<DmRecipient> object) => object.map((r) => r.toJson()).toList();
 }
 
 @JsonSerializable(fieldRename: FieldRename.snake)