Compare protocols case-insensitively

[ryu-sato]

This PR allows the protocol header of the "101 Switching Protocols" response received from the Websocket server to be compared case-insensitive.

Case-insensitive comparison is because the RFC states that it should be done so.

ref. https://www.rfc-editor.org/rfc/rfc9110.html#name-upgrade

Although protocol names are registered with a preferred case, recipients SHOULD use case-insensitive comparison when matching each protocol-name to supported protocols.

Types of Changes

• Improvement

Contribution

• [x] I tested my changes locally.
• [x] I agree to the Developer's Certificate of Origin 1.1.