CVE-2020-15234: Redirect URL matching ignores character casing

Impact

Before version v0.34.1, the OAuth 2.0 Client's registered redirect URLs and the redirect URL provided at the OAuth2 Authorization Endpoint where compared using strings.ToLower while they should have been compared with a simple string match:

1. Registering a client with allowed redirect URL https://example.com/callback
2. Performing OAuth2 flow and requesting redirect URL https://example.com/CALLBACK
3. Instead of an error (invalid redirect URL), the browser is redirected to https://example.com/CALLBACK with a potentially successful OAuth2 response, depending on the state of the overall OAuth2 flow (the user might still deny the request for example).