Fix missing title on iOS notifications

[zbarbuto]

Description

Expects a 'title' property https://developer.apple.com/library/content/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/PayloadKeyReference.html#//apple_ref/doc/uid/TP40008194-CH17-SW5

The loopback push documentation hints to a messageFrom property which would show the 'From' on a push notification.

http://loopback.io/doc/en/lb3/Push-notifications.html#send-push-notifications

This change allows the 'messageFrom' to populate the 'title' field allowing notifications that render as:

<Message From>
<Message>

Which is currently supported by the GCM provider:

https://github.com/strongloop/loopback-component-push/blob/master/lib/providers/gcm.js#L93

Also adds missing check for the 'body' property which should have been tested previously.

Checklist

• [x] New tests added or existing tests modified to cover all changes
• [x] Code conforms with the style guide

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[zbarbuto]

Not stale - still waiting for review

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[zbarbuto]

Not stale - still waiting for review

[dhmlau]

@raymondfeng , could you please take a look at this PR? Thanks.

[zbarbuto]

Thanks for merging - this one was exclusive to iOS and shouldn't affect Android.