refactor(TestAgent): removed the host param when creating Test object

`Test` constructor has already removed the host param in de056d2

Author: lamweili

lib/agent.js

@@ -39,7 +39,7 @@ TestAgent.prototype.host = function(host) {
 // override HTTP verb methods
 methods.forEach(function(method) {
   TestAgent.prototype[method] = function(url, fn) { // eslint-disable-line no-unused-vars
-    const req = new Test(this.app, method.toUpperCase(), url, this._host);
+    const req = new Test(this.app, method.toUpperCase(), url);
 
     if (this._host) {
       req.set('host', this._host);