Node.js: returning "this" from writeHead() to allow chaining.

In Node.js version 11.10.0 and later, the writeHead() function returns "this".

src/nodejs/unit-http/http_server.js

@@ -195,6 +195,8 @@ function writeHead(statusCode, reason, obj) {
             }
         }
     }
+
+    return this;
 };
 
 /*