Bad Exception fixed

When you call: 
```
$client->voice()->agents()->openTicket(['agent_id' => $agent_id,'ticket_id' => $ticket_id]); 
```

The API calls the channels/voice/agents/'.$params['agent_id'].'/tickets/'.$params['ticket_id'].'/display.json endpoint successfully and returns a 200 code, but beacouse it returns no data, the method was throwing an ResponseException.
Merge pull request #52 from freshworkstudio/master

Bad Exception fixed