Fix invalid data was sent in case of multiple user tokens

The method post_notification_to_users used a different structured payload when sending data to the server.

Currently the Push to Devices serves expects the notification data to be directly in the JSON-formatted body (not nested under a "notication_data" key).
See https://github.com/lloydmeta/push_to_devices/blob/master/app/controllers/users.rb#L93

This commit fixes this issue by using the same structure as the method for a single user token.