Accept case-insensitive URI schemes in parse_url

[xop01]

Summary

• parse_url accepted a scheme only when every character was already in a-z. RFC 3986 section 3.1 says the scheme is case-insensitive, so HTTP:// and Https:// are the same schemes as http:// and https://.
• set_follow_location(true) therefore refused a valid Location, returned a null response, and reported Error::Unknown because the redirect failed without setting error.
• The scheme is now stored in lowercase. h2:// and other non-alpha schemes are still rejected. Client and WebSocket constructors that compare against https / wss pick up HTTPS:// and WSS:// through the same parser.

Test plan

• parse_url("HTTP://example.com/path") yields scheme http, and h2:// is still rejected
• Local client/server: Location: HTTP://127.0.0.1:<port>/b is followed and returns 200
• ParseUrlTest.VariousPatterns
• RedirectLocationTest.FollowsUppercaseScheme

[yhirose]

Thanks for the PR. Rejecting an uppercase scheme in parse_url is deliberate and pinned by an existing test. I'd rather not loosen it without a concrete report from a real server. Closing.