change PayloadAlert to allow an alert which doesn't have any bodies

[co3k]

The current implementation of PayloadAlert requires "body", but the alert attribute of payload doesn't require it like the following (this example is from https://developer.apple.com/library/ios/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/Chapters/ApplePushService.html):

{
    "aps" : {
        "alert" : {
            "loc-key" : "GAME_PLAY_REQUEST_FORMAT",
            "loc-args" : [ "Jenna", "Frank"]
        },
        "sound" : "chime"
    },
    "acme" : "foo"
}

We can pass None to "body" of course but it uses wasteful bytes of payload. It is not good since the payload has length limitation.

[co3k]

My patch allows empty PayloadAlert as a result. Maybe it should be denied (by raising exceptions). Any comments?

[hagino3000]

:+1: