do not unbox single value if multiple mode

this is the odd special case referenced in #1014

src/select-menu.js

@@ -113,7 +113,7 @@ dc.selectMenu = function (parent, chartGroup) {
         // check if only prompt option is selected
         if (values.length === 1 && values[0] === '') {
             values = null;
-        } else if (values.length === 1) {
+        } else if (!_multiple && values.length === 1) {
             values = values[0];
         }
         _chart.onChange(values);