WebSocket: Improved validation and error reporting

What's Changed

• Fix macro usage by @DmitriyMusatkin in #411
• Revise "Better helper functions for querying header values (#412)" by @graebm in #414
• Validate websocket handshake response by @graebm in #410
• More websocket handshake validation by @graebm in #415
• improve websocket error reporting by @graebm in #416
• Websocket automatically responds to PING with PONG by @graebm in #417
• Websocket: Validate UTF-8 in text payloads by @graebm in #418

Full Changelog: v0.7.0...v0.7.1