Fix oauth2 for net-smtp 0.4.0

[wnm]

In net-smtp 0.4.0, auth_* methods are moved out from the Net::SMTP class, see this PR. Instead you should create a new class that inherits from Net::SMTP::Authenticator. There is a PR already merged that adds a xoauth2 class to net-smtp. Until it gets released with a new version of net-smtp, lets add it to mail_xoauth2?

[wnm]

Not 100% sure if a new mail_xoauth gem version with this fix is the best way to go... if anyone gets a ArgumentError: wrong authentication type xoauth2 error after upgrading net-smtp to 0.4.0, the easiest fix is to add a xoauth2 autenticater class to /config/initializers/auth_xoauth2.rb:

class Net::SMTP
  class AuthXoauth2 < Net::SMTP::Authenticator
    auth_type :xoauth2

    def auth(user, secret)
      token = xoauth2_string(user, secret)

      finish("AUTH XOAUTH2 #{base64_encode(token)}")
    end

    private

    def xoauth2_string(user, secret)
      "user=#{user}\1auth=Bearer #{secret}\1\1"
    end
  end
end

[fjaeger]

@wnm As net-smtp version 0.5.0 is now released, how do you want to proceed with this PR?

[wnm]

thanks @fjaeger, yes I think this is no longer needed