add store filter for collection to load correct tax_class_id when called in admin area

[ramzi-attoui]

When multisite architecture is implemented and tax_class_id is set per website. The global tax_class_id is sent to avatax. Added a store filter for both functions OnePica_AvaTax_Model_Service_Avatax_Invoice::invoice() and OnePica_AvaTax_Model_Service_Avatax_Invoice::creditmemo() to set up the correct store_id instead of the admin store.

The bug doesn't appear when estimating tax because the right front store will be set automatically (see Mage_Catalog_Model_Resource_Product_Collection ::_prepareProductLimitationFilters())