Allows messages with empty "aps" property.

While playing around with MDM service and ApnsPHP, I notice that mdmd daemon on the supervised device is not happy with empty "aps" property.
BTW, the APNS documentation says that the "aps" is reserved but not mandatory: in fact, MDM service doesn't need it at all. And omitting the empty dictionary can save few bytes which can become big when you manage thousands of devices.
So I propose this patch to allow messages with no "aps" property.

Regards.