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. 
Well, if you type-hinted the argument as suggested by @staabm, you wouldn't have to add this check at all.