fix(#233): make post_logout_redirect_uri optional

[markbrockhoff]

Closes #233

PR Checklist

• I have run npm test locally and all tests are passing.
• I have added/updated tests for any new behavior.
• If this is a significant change, an issue has already been created where the problem / solution was discussed: [N/A, or add link to issue here]

PR Description

Instead of throwing an error if the post logout redirect url isn't present no redirect will happen but instead a simple page with the text "Logout successful" will be returned. It can be used to verify the logout e.g. during integration tests.

[nulltoken]

@poveden I'm not really sure about the proposal from a pure spec standpoint.

You know better than me. What's your opinion about this?

More context available in #233