fix bug in old input check

Author: taylorotwell

src/Illuminate/Foundation/Testing/TestResponse.php

@@ -997,7 +997,7 @@ public function assertSessionHasInput($key, $value = null)
 
         if (is_null($value)) {
             PHPUnit::assertTrue(
-                $this->session()->getOldInput($key),
+                $this->session()->hasOldInput($key),
                 "Session is missing expected key [{$key}]."
             );
         } elseif ($value instanceof Closure) {