Fix parsing GetProductCategoriesForASIN response

The amazon mws GetProductCategoriesForASIN api call
returns multiple <Self> nodes (if available), but
the code returned only the last XML node.
fixup! Fix parsing GetProductCategoriesForASIN response
Merge pull request #2024 from ckrybus/fix-mws-get-product-categories

Fix parsing GetProductCategoriesForASIN response. Fixes #2024.