Verify success from the response content when from API response code is ...

...HTTP 200 OK

Sample 200 OK responses from Mandrill:

	[
	    {
	        "email": "name@domain.com",
	        "status": "sent",
	        "_id": "e985ec5dbced4fcd9eae1226ec475a55",
	        "reject_reason": null
	    }
	]

	[
	    {
	        "email": "name@domain.com",
	        "status": "rejected",
	        "_id": "e985ec5dbced4fcd9eae1226ec475a55",
	        "reject_reason": "invalid-sender"
	    }
	]

In that case the flow needs to be changed. As shown in the sample responses, a failure may occur even when the response is `200 OK`. Currently `handle()` is only called with an expectation that it will definitely raise an error, which could be re-purposed to handle the response and decide on further action rather then the `Mailer` class doing it.