WebSocket server responds incorrectly if connection is closed without a status code

If a browser-based client connects to h's WebSocket server using the `WebSocket` API and later closes the connection by calling `close()` without any arguments, h's WebSocket server will respond with an invalid close frame with a status code of 1005 which will result in Safari and Chrome displaying an error in the console.

Client issue: https://github.com/hypothesis/client/pull/1941

**Steps to reproduce:**

Save the following HTML file locally, open it and click the "Close connection" button.

```html

Not connected
Close connection

Clicking the "Close connection" button will display an error in the
browser console in Safari and Chrome, and the WS will receive a
"close" event with a status of 1006 indicating an abnormal disconnection.

In Firefox a status of 1005 is reported.

const btn = document.querySelector('#closeButton');
const status = document.querySelector('#wsStatus');

const ws = new WebSocket('ws://localhost:5001/ws');
ws.onopen = () => status.textContent = 'Connected';
ws.onclose = e => status.textContent = `Closed (status: ${e.code})`;

btn.onclick = () => ws.close();

```

**Expected:** No error message in the browser console and a close status of 1005.
**Actual:** Safari and Chrome show an error message in the browser console and a close status of 1006.

**Notes**

The 1005 status code should never be sent over the wire. Instead this code is reserved to be synthesized by the receiving end of the connection to indicate that a close frame was received with no status code.

I believe Safari and Chrome's behavior here are correct. Firefox's behavior is an error.