lib: replace string concatenation with template

PR-URL: #16921
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>

Author: Jayashree S Kumar

lib/_http_server.js

@@ -176,7 +176,7 @@ ServerResponse.prototype.detachSocket = function detachSocket(socket) {
 };
 
 ServerResponse.prototype.writeContinue = function writeContinue(cb) {
-  this._writeRaw('HTTP/1.1 100 Continue' + CRLF + CRLF, 'ascii', cb);
+  this._writeRaw(`HTTP/1.1 100 Continue${CRLF}${CRLF}`, 'ascii', cb);
   this._sent100 = true;
 };
 
@@ -230,7 +230,7 @@ function writeHead(statusCode, reason, obj) {
   if (checkInvalidHeaderChar(this.statusMessage))
     throw new errors.Error('ERR_INVALID_CHAR', 'statusMessage');
 
-  var statusLine = 'HTTP/1.1 ' + statusCode + ' ' + this.statusMessage + CRLF;
+  var statusLine = `HTTP/1.1 ${statusCode} ${this.statusMessage}${CRLF}`;
 
   if (statusCode === 204 || statusCode === 304 ||
       (statusCode >= 100 && statusCode <= 199)) {
@@ -456,7 +456,7 @@ function onParserExecute(server, socket, parser, state, ret, d) {
 }
 
 const badRequestResponse = Buffer.from(
-  'HTTP/1.1 400 ' + STATUS_CODES[400] + CRLF + CRLF, 'ascii'
+  `HTTP/1.1 400 ${STATUS_CODES[400]}${CRLF}${CRLF}`, 'ascii'
 );
 function socketOnError(e) {
   // Ignore further errors