apns push notification payload builder method

[Serafino Sorleti]

Hi Monnand,

The following method of apns.go

func toAPNSPayload(n *Notification) ([]byte, error)

could create an "alert" dictionary containing only the "body" value when only the msg params of push request has a value. 

As suggested by Apple documentation it is better to use only a string as "alert" value in such a case. It is a simple fix but it is not technically an issue (even if a couple of bytes could be saved for iOS6/7 256byte notifications) so i preferred to post this message here instead on github. 

Thanks in advance for reading my post.

Serafino.