Fix VAPID default values (merge pull requests)

[collimarco]

I have opened this two pull requests to improve / fix VAPID default values:

• Change VAPID expiration from 24h to 12h #101
• Fix wrong default value for VAPID subject #100

I think those changes will benefit anyone who is using this gem (as explained inside each pull request).

[maltesa]

Would be great to see this merged 👍