Check wc_checkout_params.is_checkout against string '1' instead of int 1

This was causing `’init_checkout’` not to be triggered on page load. I
noticed this when the taxes for an order weren’t being shown on the
checkout page until changing the billing address.

Introduced in b85b1ab03b

assets/js/frontend/checkout.js

@@ -391,7 +391,7 @@ jQuery( function( $ ) {
 	});
 
 	// Update on page load
-	if ( wc_checkout_params.is_checkout === 1 ) {
+	if ( wc_checkout_params.is_checkout === '1' ) {
 		$( 'body' ).trigger( 'init_checkout' );
 	}