Support custom request attributes without loosing the information provided by grailsEvents

When the deep flag is not set when calling jQuerys extend the user has to re-add the topic-header information by himself. This maybe a commons use case when trying to access custom request parameters in the browserFilter closure. 

```
grailsEvents.on('myTopic', function(data) {
 // consume response ...
}, {headers: {aUserToken: '1234'}});
```

Use deep flag when extending the request object created by grailsEvents
to allow the user to extend/override request attributes like headers
without loosing information provided by grailsEvents.