Use inherited writeHead method signature

The `writeHead` method gets passed through to the native `response.writeHead`. The `Response` interface extends `http.ServerResponse` which defined the `writeHead` method with different arguments. For maintainability reasons, we should just use the inherited method. Also, the current definition makes this method useless (`writeHead` expects at lease one argument).

types/restify/index.d.ts

@@ -919,15 +919,6 @@ export interface Response extends http.ServerResponse {
      */
     toString(): string;
 
-    /**
-     * pass through to native response.writeHead().
-     * @public
-     * @function writeHead
-     * @emits    header
-     * @returns  {undefined}
-     */
-    writeHead(): void;
-
     /** redirect is sugar method for redirecting.
      * res.redirect(301, 'www.foo.com', next);
      * `next` is mandatory, to complete the response and trigger audit logger.

types/restify/restify-tests.ts

@@ -95,7 +95,10 @@ function send(req: restify.Request, res: restify.Response, next: restify.Next) {
     res.id === 'test';
 
     res.send('hello ' + req.params.name);
-    res.writeHead();
+    res.writeHead(200);
+    res.writeHead(200, {
+        "Content-Type": "application/json"
+    });
     return next();
 }