fix: websocket conns do not require Connection: upgrade header

[mccutchen]

It turns out that the WebSocket API provided by browsers does not actually send the Connection: upgrade header that our websocket implementation (wrongly?) requires, so here we're dropping that requirement.

Tested with both https://websocketking.com/ and by pasting this ChatGPT-generated code into the Firefox web inspector's console:

// Open a WebSocket connection
const socket = new WebSocket('ws://localhost:8080/websocket/echo');

// Event handler for when the connection is established
socket.addEventListener('open', (event) => {
  console.log('WebSocket connection opened:', event);

  // Send test messages
  const testMessages = ['Hello, server!', 'How are you?', 'Testing WebSocket'];
  testMessages.forEach((message, index) => {
    setTimeout(() => {
      console.log('Sending message:', message);
      socket.send(message);
    }, index * 1000); // Delay messages by 1 second intervals
  });
});

// Event handler for receiving messages
socket.addEventListener('message', (event) => {
  console.log('Received message:', event.data);
});

// Event handler for errors
socket.addEventListener('error', (event) => {
  console.error('WebSocket error:', event);
});

// Event handler for when the connection is closed
socket.addEventListener('close', (event) => {
  console.log('WebSocket connection closed:', event);
});

[codecov]

Codecov Report

Merging #161 (4cbb52f) into main (21c68b8) will decrease coverage by 0.01%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #161      +/-   ##
==========================================
- Coverage   95.05%   95.04%   -0.01%     
==========================================
  Files           9        9              
  Lines        2122     2119       -3     
==========================================
- Hits         2017     2014       -3     
  Misses         71       71              
  Partials       34       34              

Files	Coverage Δ
httpbin/websocket/websocket.go	77.28% <ø> (-0.23%)	⬇️