Fixed seralization error in responsetransporter, when changes containing dates.

When a deadline of a multiclient task is modified before accepted. The copy from one mandant to the other fails with the following  traceback.
```
  Module zope.publisher.publish, line 113, in debug_call
  Module opengever.task.transporter, line 231, in render
  Module opengever.task.transporter, line 107, in extract_responses
  Module json, line 231, in dumps
  Module json.encoder, line 201, in encode
  Module json.encoder, line 264, in iterencode
  Module json.encoder, line 178, in default
TypeError: datetime.date(2014, 8, 5) is not JSON serializable
```

The reason for this problem was that the ResponseTransporter encoded the values of dict not separately.

@lukasgraf please have a look...