move extension check into class extend method
move extension check into class extend method

smelukov/basisjs,basisjs/basisjs,tyanas/basisjs,fateevv/basisjs,smelukov/basisjs,tyanas/basisjs,tyanas/basisjs,istrel/basisjs,istrel/basisjs,istrel/basisjs,basisjs/basisjs,fateevv/basisjs,smelukov/basisjs,fateevv/basisjs,basisjs/basisjs