Fix IE Download via SSL bug and fix http file over SSL x-sendfile bug

Download of files does not work with Internet Explorer when My Account is SSL.

http://support.microsoft.com/kb/316431
http://support.microsoft.com/kb/812935

The problem is that `woocommerce_download_product()` calls `nocache_headers()` which sets both the Cache-Control and Pragma headers. For IE, the Pragma header should not be set or the download fails with file not found.

This needs to be fixed ASAP, IMO.

I also fixed the x-sendfile case where the file is http but the My Account is served over SSL. I thought my previous fix handled this, but I didn't realize that WooCommerce uses a filter to override the behaviour of `$site_url = site_url( '', $scheme );` Even though I forcibly passed a scheme of http, WC forced it to https.