Proxy WebSocket connections with error handling

[Hermholtz]

Properly proxy WebSocket "upgrade" connections. Without this change any attempt to use http-server as a proxy for WebSocket does not work. The code also handles error cases when WebSocket server abruptly closes connection (or a network device dies between client and server). Other PRs fixing the same issue cause http-server to throw error and exit on WebSocket connection errors.

It's a useful addition to http-server for people developing WebSocket applications. There's no WebSocket proxy for Node.js easy to use from command line. This pull request makes http-server more versatile and helps local dev workflows.

The change is as minimal as possible, no code style changes, no whitespace changes etc. Please merge.

Relevant issues

Fixes #142.

Contributor checklist

Note: cannot provide unit tests for this change.
Note: this doesn't add any new command line options.

• Provide tests for the changes (unless documentation-only)
• Documented any new features, CLI switches, etc. (if applicable)
  • Server --help output
  • README.md
  • doc/http-server.1 (use the same format as other entries)
• The pull request is being made against the master branch

Maintainer checklist

• Assign a version triage tag
• Approve tests if applicable