Enable non-ascii characters in JWT

[tom-010]

Problem: When processing the response from an OpenID-Provider, the string is not decoded using UTF-8. Therefore, no non-ascii characters like german "Umlaute" (like ä, ü, ö) are possible in the JWT and therefore in the name in the JWT (like Jürgen).

Fix: in lib/src/http_util.dart _processResponse: before calling json.decode of the response, first decode it via utf8.decode.

Result: Umlaute and other non-ascii characters are now possible in the JWT and its fields like the name.

Pull Request: I already created a pull-request for it: Bugfix: Enable non-ascii characters in JWT