oauth2: add www-authenticate header in case of unauthorized error for userinfo endpoint

[bayansar]

Related issue

Here is the issue that this PR is trying to fix /userinfo endpoint misses www-authenticate header for 401 response #1827

Proposed changes

As far as I understood, apparently there are two cases that /userinfo API returns 401 error. One is if introspection API returns 401 and the other one is if token type is not fosite.AccessToken. Thus, I covered these two cases by adding www-authenticate header with reason coming from RFC6749Error

I've also added a new test case but needed small changes to make it happen.

This is my first PR, and I'm not familiar with the codebase. I'd be glad to hear better ways to do these changes if any.

Checklist

• I have read the contributing guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security. vulnerability, I
  confirm that I got green light (please contact
  security@ory.sh) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further comments

[CLAassistant]

All committers have signed the CLA.

[aeneasr]

Good job and thank you for the contribution :)

[heinzburgstaller]

This is the same for then /oauth2/token endpoint. If there is a 401 response the response header "WWW-Authenticate" is missing

[aeneasr]

Contributions welcomed!