fix check of invalid jwt.encode parameters from twitcher.oauth2.CustomTokenValidator

[fmigneault]

An incorrectly configured token validator (such as in the tests) would result in failure when invoking the token generation. They will be raised upon initialization to allow early catch of the problem.

FAILED tests/functional/test_oauth2_app.py::OAuth2AppTests::test_compute_with_header - TypeError: Issuer (iss) must be a string.
FAILED tests/functional/test_oauth2_app.py::OAuth2AppTests::test_compute_with_param - TypeError: Issuer (iss) must be a string.

Error seen during #152 CI runs.

[cehbrecht]

... when tests pass :)

[fmigneault]

@cehbrecht
Do you want me to create a PR that combines everything? They are interlocking each other because of pkg_resource #149

I also see that CodeCov complains about a missing token.
I sent a request to accept in bird-house org. I do not have access to enable it myself.