Time to live not serialised when set to 0

[milesich]

You may want to set time to live to 0 when you want the notification to be delivered now or never.

Currently when you set time to live to 0 the parameter is not serialised and thus the default value for time to live is 4 weeks.

[erickskrauch]

This issue has been resolved in my fork.
https://github.com/erickskrauch/fcm