Cart fixes

Testing the cart functionality I found a few issues:
 - cart clear wasn't working due to incorrect casing in localStorage variable
 - with IPN simulator sometimes sent null shipping, fee and tax, so I avoid that error
 - Price is incorrectly assigned total
 - finally, when I use special characters in Spanish for the SKU I would get a null value. After digging around I found that although we set and send the information in UTF8 we receive it in ISO8859, and the the json_encode eliminates it. I force a UTF8 conversion.