feat(form): accept 1 in addition to true as checked value for checkboxes

When using the behavior set value or set values, checkbox elements were only supporting a real boolean true to check any checkbox.
This PR additionally supports the integer value 1 to have a checkbox checked now.

Closes #987

src/definitions/behaviors/form.js

@@ -1053,7 +1053,7 @@ $.fn.form = function(parameters) {
                 }
                 else if(isCheckbox) {
                   module.verbose('Setting checkbox value', value, $element);
-                  if(value === true) {
+                  if(value === true || value === 1) {
                     $element.checkbox('check');
                   }
                   else {