Elements with the "multiple" but not "multiple_toggle" property (e.g. checkboxes) don't have their default values saved

If you set up an options element instance with #multiple set to TRUE but no #multiple_toggle, the element is treated as if it does not have the #multiple property in the value callback function, which results in the default value of the element not being saved correctly.

This issue arises in Form Builder, for the checkbox element (which forces #multiple to be TRUE but does not want to provide a UI to configure it, so it does not use #multiple_toggle).

The attached patch fixes it and should apply to both 6.x and 7.x.