Fix UrlGenerator class_exists on interface (#30)

Author: bumbummen99

src/UrlGenerator.php

@@ -20,7 +20,7 @@ class UrlGenerator extends BaseUrlGenerator
      */
     public function __construct($routes, Request $request, $assetRoot = null)
     {
-        if (!$routes instanceof RouteCollection && !(class_exists('Illuminate\Routing\RouteCollectionInterface') && is_subclass_of($routes, 'Illuminate\Routing\RouteCollectionInterface'))) {
+        if (!$routes instanceof RouteCollection && !(interface_exists('\Illuminate\Routing\RouteCollectionInterface') && is_subclass_of($routes, '\Illuminate\Routing\RouteCollectionInterface'))) {
             throw new \InvalidArgumentException('The $routes parameter has to be of type RouteCollection or RouteCollectionInterface for L6+.');
         }