client/lib/cart/store/cart-synchronizer.js
function castProductIDsToNumbers( cartItems ) {
	return cartItems.map( function( item ) {
		return assign( {}, item, { product_id: parseInt( item.product_id, 10 ) } );
	} );
}
function castProductIDsToNumbers( cartItems ) {
	return cartItems.map( function( item ) {
		return assign( {}, item, { product_id: parseInt( item.product_id, 10 ) } );
	} );
}
FIXME: Temporary fix to cast string product IDs to numbers. There is a bug with the API where it sometimes returns product IDs as strings.