Allow non-keyword keys in selects.

Currently, keys in selects have to be keywords, i.e…
```clojure
[:ul.list-group {:field :single-select :id :pick-one}
  [:li.list-group-item {:key :foo} "foo"]
  [:li.list-group-item {:key :bar} "bar"]
  [:li.list-group-item {:key :baz} "baz"]]
```
…is allowed, but not…
```clojure
[:ul.list-group {:field :single-select :id :pick-one}
  [:li.list-group-item {:key :foo} "foo"]
  [:li.list-group-item {:key 123} "bar"]
  [:li.list-group-item {:key "baz"} "baz"]]
```
While using keywords as keys is more idiomatic, using other types can be convenient (e.g. when populating forms from JSON data).

This pull request removes this constraint.