oauthex/auth_meta.go: fixes metadata issuer comparison to ignore
trailing slash difference
Google's gmail MCP server returns `https://accounts.google.com/` as the
Authorization Server in the Protected Resource Metadata, but returns
`https://accounts.google.com` in the Auth Server Metadata. This causes a
comparison failure per rfc8414 section-3.3: Authorization Server
Metadata Validation. The authorization server URL is meant to be a base
to build other URLs from, it does not seem necessary to enforce that the
trailing slash (or lack thereof) matches with the issuer URL. Please see
#953 for more details.
Fixes#953
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Co-authored-by: Guglielmo Colombo <guglielmoc@google.com>
da3535b678