Fix www-authenticate header parsing bug when refreshing tokens

[JonMerlevede]

Description

This PR applies a minimal change to fix the issue where the OAuth refresh token flow results in the "Header info didn't have x_token_server" error:

trino.exceptions.TrinoAuthError: Error: header info didn't have x_token_server

This error was introduced in #467. The regression was not captured by the test test_oauth2_refresh_token_flow; Trino returns multi-valued Www-Authenticate headers while PostStatementCallback does not, and the new parsing logic does not handle this appropriately. This PR also updates the test.

Non-technical explanation

This PR fixes the issue where Trino returns "Header info didn't have x_token_server" after some time and starts working again after a longer time.

Release notes

( ) This is not user-visible or docs only and no release notes are required.
( ) Release notes are required, please propose a release note for me.
(x) Release notes are required, with the following suggested text:

* Fixes "header info didn't have x_token_server" error. ({issue}`484`)

[cla-bot]

Thank you for your pull request and welcome to the Trino community. We require contributors to sign our Contributor License Agreement, and we don't seem to have you on file. Continue to work with us on the review and improvements in this PR, and submit the signed CLA to cla@trino.io. Photos, scans, or digitally-signed PDF files are all suitable. Processing may take a few days. The CLA needs to be on file before we merge your changes. For more information, see https://github.com/trinodb/cla

[hashhar]

please see ci complaints and squash the commits into one Fix parsing of www-authenticate header since it's a single logical change (tests + impl go together).

Thanks for the PR.

[hashhar]

LGTM % ci and commit

[cla-bot]

Thank you for your pull request and welcome to the Trino community. We require contributors to sign our Contributor License Agreement, and we don't seem to have you on file. Continue to work with us on the review and improvements in this PR, and submit the signed CLA to cla@trino.io. Photos, scans, or digitally-signed PDF files are all suitable. Processing may take a few days. The CLA needs to be on file before we merge your changes. For more information, see https://github.com/trinodb/cla

[cla-bot]

Thank you for your pull request and welcome to the Trino community. We require contributors to sign our Contributor License Agreement, and we don't seem to have you on file. Continue to work with us on the review and improvements in this PR, and submit the signed CLA to cla@trino.io. Photos, scans, or digitally-signed PDF files are all suitable. Processing may take a few days. The CLA needs to be on file before we merge your changes. For more information, see https://github.com/trinodb/cla

[hashhar]

reworded the commit message. Will merge on CI done.