Callback for Mandril Handler

The following code can be used to silence the curl response. It also allows you to pass in a call back function which can then be used to process the response from mandrill. 
We were using this project in a website and upon sending an email to mandrill, the response was being echoed out on our site. 
Can you remove this `if` statement and always et `CURLOPT_RETURNTRANSFER` to `1` (even if `responseHandler` is `null`)? I really can't see any reason why anybody would want the API response directly output. Should have really included this line when I first added the handler.