http: use cached '1.1' http version string

lib/_http_common.js

@@ -64,6 +64,8 @@
   this._url += url;
 }
 
+const HTTP_VERSION_1_1 = '1.1';
+
 // `headers` and `url` are set only if .onHeaders() has not been called for
 // this request.
 // `url` is not set for response parsers but that's not applicable here since
@@ -91,7 +93,9 @@
   const incoming = parser.incoming = new ParserIncomingMessage(socket);
   incoming.httpVersionMajor = versionMajor;
   incoming.httpVersionMinor = versionMinor;
-  incoming.httpVersion = `${versionMajor}.${versionMinor}`;
+  incoming.httpVersion = versionMajor === 1 && versionMinor === 1
+    ? HTTP_VERSION_1_1
+    : `${versionMajor}.${versionMinor}`;
   incoming.joinDuplicateHeaders = socket?.server?.joinDuplicateHeaders ||
                                   parser.joinDuplicateHeaders;
   incoming.url = url;