Adds proper fix for select/option

Related to [#9436](https://github.com/emberjs/ember.js/issues/9436)
http://www.w3.org/html/wg/drafts/html/master/forms.html#the-option-element

> selected - Whether the option is selected by default
>
> The selected attribute is a boolean attribute. It represents the default selectedness of the element.

http://www.w3.org/html/wg/drafts/html/master/infrastructure.html#boolean-attribute

> If the attribute is present, its value must either be the empty string or a value that is an ASCII case-insensitive match for the attribute's canonical name, with no leading or trailing whitespace.

Following the spec as closely as possible is what we want here.