post custom data when fetching a variation via ajax

Sometimes it is needed to add "context" to a variations ajax request, in order modify the returned object content. For instance, in cases where the variation is part of a bundled variable product, we may need to modify the price, stock status or the description of the variation according to the "context".

assets/js/frontend/add-to-cart-variation.js

@@ -116,7 +116,8 @@
 
 				if ( all_attributes_chosen ) {
 					// Get a matchihng variation via ajax
-					data.product_id = $product_id;
+					data.product_id  = $product_id;
+					data.custom_data = $form.data( 'custom_data' );
 
 					$( '.variations_form' ).block({
 						message: null,