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.