Save check box value fix
* When check boxes are saved the contents of the attribute value are used as the WPeC meta value
* When check boxes are checked/not checked if the meta value is the empty string the check box stays unchecked, otherwise a non-empty string checks the box

Resolves issue #1236
Merge pull request #1241 from JeffPyeBrook/pr-1236-save-custom-checkout-item-value

Save checkout form check box value fix