expose the Application::handleRequest method (#950)

* * expose the Application::handleRequest method to extend the context

* * minor change the handleRequest comment

lib/application.js

@@ -128,17 +128,28 @@ module.exports = class Application extends Emitter {
     if (!this.listeners('error').length) this.on('error', this.onerror);
 
     const handleRequest = (req, res) => {
-      res.statusCode = 404;
       const ctx = this.createContext(req, res);
-      const onerror = err => ctx.onerror(err);
-      const handleResponse = () => respond(ctx);
-      onFinished(res, onerror);
-      return fn(ctx).then(handleResponse).catch(onerror);
+      return this.handleRequest(ctx, fn);
     };
 
     return handleRequest;
   }
 
+  /**
+   * Handle request in callback.
+   *
+   * @api private
+   */
+
+  handleRequest(ctx, fnMiddleware) {
+    const res = ctx.res;
+    res.statusCode = 404;
+    const onerror = err => ctx.onerror(err);
+    const handleResponse = () => respond(ctx);
+    onFinished(res, onerror);
+    return fnMiddleware(ctx).then(handleResponse).catch(onerror);
+  }
+
   /**
    * Initialize a new context.
    *