Allow APNS token from feedback to be encoded with lowercase

[Nicklas Ramhöj Holtryd]

The feedback module gets called with an uppercase representation of the hex APNS token (https://github.com/chvanikoff/apns4ex/blob/master/lib/apns/worker.ex#L122). It would be nice to allow the user to configure this.

config :apns,
  callback_module:    APNS.Callback,
  token_case: :lower # defaults to :upper,
  …

Would you consider a PR to address this?

[Vladimir Reshetnikov]

Yes, please.

[Nicklas Ramhöj Holtryd]

Closing very old issue.