Switch download_url to non-SSL

Change download_url (as shown under My Account / Available Downloads) to use non-SSL (http) instead of SSL. This prevents issues with certain browsers like IE and caching security settings. I cannot see a compelling reason to serve these links over SSL.

Note that this does not change the actual link saved with the product, just the 'download handler' link WC uses.

See #2871

If someone wants SSL behavior, use `woocommerce_customer_get_downloadable_products` filter to modify these urls.