Fix WebSocket client handshake failing with non-ASCII header values

[BobaFetters]

The WebSocket client handshake was incorrectly rejecting HTTP headers containing non-ASCII bytes. This occurred because generate_request() called HeaderValue::to_str() on custom headers, which fails for any non-ASCII content.

HTTP headers are defined as octets per RFC 7230, not UTF-8 strings. This fix writes header values as raw bytes, preserving their original content.

Changes

• Replace to_str() + writeln!() with direct byte operations using extend_from_slice()
• Add test for UTF-8 encoded header values (e.g., Montréal)
• Add test for Latin-1 encoded header values (e.g., café)

Fixes #531