Make protocol upgrade case-insensitive

[rawrafox]

Description

Some clients/servers don't send the protocol upgrade header as exactly "websocket", and instead send it as for example "WebSocket".

This is allowed according to https://github.com/httpwg/http-core/pull/131 so it probably should be supported even if we never send it.

Types of Changes

• Bug fix.

Testing

• [ ] I added tests for my changes.
• [x] I tested my changes locally.
• [ ] I tested my changes in staging.
• [ ] I tested my changes in production.

[ioquatix]

We should probably add a spec for this to avoid regressions.