How to set the `aud` for localhost

[George3d6]

Chrome/Chromium require the aud parameter in the vapid claims (403 error asking for it is returned otherwise).

However just setting this to http://localhost:port doesn't seem to work (it's not recognized as a valid aud)

[George3d6]

Ups, I was mistaken here, the aud is supposed to be the actual push service origin (e.g. https://fcm.googleapis.com for chromium`) -- in this case, should this parameter be automatically extracted from the subcription info being passed?

Could make a PR with this fix if so.

[jrconlin]

we should be extracting the aud from the vapid claims, so long as it's not present (e.g. it's None not an empty string). Is that not working for you?