selectMenu doesn't work with date keys

[gordonwoodhull]

It is effectively coercing the keys to strings, because it is filtering on the value attribute of the <option> tags. It should use the original data instead of pulling from HTML attributes.

            values = selectedOptions.map(function (d) {
                return d.value;
            });

https://github.com/dc-js/dc.js/blob/develop/src/select-menu.js#L99

should use d3.select(d).datum().key instead.

(Unless there's something more idiomatic we can do - usually not an issue because we are attaching event handlers directly to the elements, but here we are listening to the parent which doesn't itself have data (should it?))

[gordonwoodhull]

Prompted by

http://stackoverflow.com/questions/38591613/how-to-create-interaction-with-selectmenu-in-dc-js