fixed AssetCollection compatibility issue

I had some problemas while using  an asset manager config as shown bellow.

     'asset_manager' => function(AssetManager $am){                               
         $css = new AssetCollection(array(                                        
               new FileAsset(base_path().'/assets/css/dev.css'),                        
               new FileAsset(base_path().'/assets/css/project.css'),             
         ));                                                                      
         $scripts = new AssetCollection(array(                                    
             new GlobAsset(base_path().'/assets/js/libs/*'),                      
             new GlobAsset(base_path().'/assets/js/*')                            
         ));                                                                      
                                                                                  
         $am->set('base_css', $css);                                              
         $am->set('base_js', $scripts);                                           
     },

CheckedAssetWriter was trying to write to a folder instead of the files. Fixed this issue by preventing writing text content directly into folders.