Shopping cart minor bug fixed

Minor software bugs which prevents product items to be added successfully to the shopping cart if the "weight" is not defined properly.

It has been discovered that add-to-cart operation will fail if the optional "weight" parameter is not correctly defined.
(for example < input type="hidden" value="" name="weight"/> ). Notice in these example the name parameter is defined, but the value parameter is not defined.
This has caused the "add-to-cart" operation to fail and subsequently the shopping cart will not be displayed.

However, it will not affect the operation of the parameters are correctly setup.

This bug has been fixed. Hence, shops which has incorrectly configured parameters should operate now.