WooCommerce Google Product Feeds 11.0.14

* Fix issue where variation ID not passed to woocommerce_gpf_elements filters
* Fix issue where incorrect product ID passed to woocommerce_gpf_elements filters
* Fix issue where is_bundle checkbox could not be unticked on a product edit screen
* Fix issue where sale price effective dates weren't output correctly on WooCommerce 3.x
* Fix warning shown generated when products have sale prices on WooCommerce 3.x
* Update identifier_exists logic. Identifiers are now required for all countries

* Fix issue with "identifier exists" option not showing selected value correctly on settings form
* Change mime-type of bing feed to fix issues when running behind CloudFlare
* Fix issues where variation options were missing from variation titles in the feed

* Fix issue where descriptions weren't always passed through the_content filter
* Fix issue where URL-based category filtering didn't work with permalink-style feed URLs
* Remove some unused code