Product Search Constraint on CountBy...methods

If a search constraint like featuresId or categorieId is applied, the CountBy...methods in the ProductSearchSession fail.

Error: mainProductId field is missing.

Attached is a patch that fixes the problem.

Test:

1. Enable LayeredNavigation function in ecommerce module.
2. Add feature "TEXT_BLACK" to product GZ-1000 as standard feature
3. Add "ProductSearchSession.searchAddConstraint(new ProductSearch.FeatureConstraint("TEXT_BLACK", true), session);" in LayeredNavigation.groovy before "result = ProductSearchSession.getProductSearchResult(request, delegator, prodCatalogId);"
4. Navigate to the product via category menu.
5. --> GZ-1000 should not be visible + no error message