Support optional id_tokens in refresh responses (#335)

* OIDC Token Refresh works without id_tokens

Addresses https://github.com/pusher/oauth2_proxy/issues/318

Refactoring the OIDC provider so that the refresh process works when there are
no id_tokens present in the response. Added unit tests to the oidc_test.go to prove
the redeem and refresh still work.

The expiry time of the session is now taken from the outh token expiry and not
the id_token (preventing stale access_tokens in sessions).

* Refactoring the to use a KeySetStub in the oidc_test.go. This allows the
elimination of the slightly contrived function passing elements used
previously. (This change is being applied to address the bug #318)

* Changes as per the PR comments and preparing for 5.x release

* Fixup changelog

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>