Add image/png header for plain renderer
Add image/png header for plain renderer

manueltimita/Windshaft,HydroLogic/Windshaft,wsw0108/Windshaft,HydroLogic/Windshaft,wsw0108/Windshaft,manueltimita/Windshaft,HydroLogic/Windshaft,wsw0108/Windshaft,manueltimita/Windshaft,CartoDB/Windshaft,CartoDB/Windshaft,CartoDB/Windshaft