[http2] Fix Http2Adapter not redirecting for relative URLs

[kuhnroyal]

Fixes a redirect bug discovered during https://github.com/cfug/dio/pull/2077 where the adapter would fail to properly redirect if the location header contains a relative URL.

New Pull Request Checklist

• [x] I have read the Documentation
• [x] I have searched for a similar pull request in the project and found none
• [x] I have updated this branch with the latest main branch to avoid conflicts (via merge from master or rebase)
• [x] I have added the required tests to prove the fix/feature I'm adding
• [x] I have updated the documentation (if necessary)
• [x] I have run the tests without failures
• [x] I have updated the CHANGELOG.md in the corresponding package

Additional context and info (if any)

[AlexV525]

@kuhnroyal Still WIP?

[kuhnroyal]

Updated