Select options: only set selected attribute when its true

The `selected` attribute for an `option` element was always being set to either `true` or `false`. This breaks the selected attribute rendering (at least in Chrome). Only one option should ever have the `selected` attribute.

See http://jsbin.com/suwojito/1/

Output:

```clojure
user=> (build-opt-tag "foo" "bar" "foo")
[:option {:selected "true", :value "foo"} "bar"]
user=> (build-opt-tag "foo" "bar" "quux")
[:option {:value "foo"} "bar"]
```