http: send connection: close when closing conn

HTTP/1.1 mandates connections which do not support keep-alive and
close the connection send the connection: close header, see
https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.10

This page also provides more information:
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Connection

I understand that HTTP/1.1 defaults to keep-alive - and that the
Connection: close header is required when closing a connection.

This adds the Connection: close header in the 400 responses
sent on client errors.

PR-URL: #26467

lib/_http_server.js

@@ -501,7 +501,8 @@ function onParserExecute(server, socket, parser, state, ret) {
 }
 
 const badRequestResponse = Buffer.from(
-  `HTTP/1.1 400 ${STATUS_CODES[400]}${CRLF}${CRLF}`, 'ascii'
+  `HTTP/1.1 400 ${STATUS_CODES[400]}${CRLF}` +
+  `Connection: close${CRLF}${CRLF}`, 'ascii'
 );
 function socketOnError(e) {
   // Ignore further errors

test/parallel/test-http-blank-header.js

@@ -52,7 +52,9 @@ server.listen(0, common.mustCall(() => {
     received += data.toString();
   }));
   c.on('end', common.mustCall(() => {
-    assert.strictEqual('HTTP/1.1 400 Bad Request\r\n\r\n', received);
+    assert.strictEqual(received,
+                       'HTTP/1.1 400 Bad Request\r\n' +
+                       'Connection: close\r\n\r\n');
     c.end();
   }));
   c.on('close', common.mustCall(() => server.close()));

test/parallel/test-http-server-destroy-socket-on-client-error.js

@@ -37,7 +37,9 @@ server.listen(0, () => {
   });
 
   socket.on('end', mustCall(() => {
-    const expected = Buffer.from('HTTP/1.1 400 Bad Request\r\n\r\n');
+    const expected = Buffer.from(
+      'HTTP/1.1 400 Bad Request\r\nConnection: close\r\n\r\n'
+    );
     assert(Buffer.concat(chunks).equals(expected));
 
     server.close();