Fix using module.function deprecation warning.

[crova]

Hello there, how is it going?

First of all, thanks for the great library.

While bumping our project to elixir 1.17 we got the following deprecation warning:

warning: using module.function() notation (with parentheses) to fetch map field :__struct__ is deprecated, you must remove the parentheses: map.field
  (ex_oauth2_provider 0.5.7) lib/ex_oauth2_provider/mixin/expirable.ex:45: ExOauth2Provider.Mixin.Expirable.is_expired?/1
  (ex_oauth2_provider 0.5.7) lib/ex_oauth2_provider/access_tokens/access_tokens.ex:234: ExOauth2Provider.AccessTokens.is_accessible?/1
  (ex_oauth2_provider 0.5.7) lib/ex_oauth2_provider.ex:86: ExOauth2Provider.validate_access_token/1
  (ex_oauth2_provider 0.5.7) lib/ex_oauth2_provider.ex:58: ExOauth2Provider.authenticate_token/2
  (ex_oauth2_provider 0.5.7) lib/ex_oauth2_provider/plug/verify_header.ex:78: ExOauth2Provider.Plug.VerifyHeader.verify_token/4

This takes care of it.

Let me know if you see any issues.

Cheers.

[heroinbob]

@danschultzer is there any chance of this getting merged and released?