test: guard write to proxy client if proxy connection is ended

In the testing proxy server for proxy client tests, the proxy
client might have already closed the connection when the upstream
connection fails. In that case, there's no need for the proxy
server to inform the proxy client about the error.

Author: joyeecheung

test/common/proxy-server.js

@@ -91,8 +91,10 @@ exports.createProxyServer = function(options = {}) {
 
     proxyReq.on('error', (err) => {
       logs.push({ error: err, source: 'proxy request' });
-      res.write('HTTP/1.1 500 Connection Error\r\n\r\n');
-      res.end('Proxy error: ' + err.message);
+      if (!res.writableEnded) {
+        res.write('HTTP/1.1 500 Connection Error\r\n\r\n');
+        res.end('Proxy error: ' + err.message);
+      }
     });
   });