Update taxonomy collection to return indexed array

Whilst testing some code in the backbone JS client (cc @tlovett1) I came accross an issue with the `/posts/types/post/taxonomies/` endpoint.

A taxonomies collection requres the response to be an array of taxonomies, however because this is an associative array (using taxonomy slug), `json_encode` converts it to an object - which backbone doesn't like.

An alternative would be to handle this with a custom parse method on the backbone collection (see code below) however I think that it should be fixed here - although feel free to tell me where to go...!

````
        // Correctly format response.
        parse: function (response) {
            return jQuery.map( response, function( value ) {
                return [value]
            } );
        }
````

If this should be fixed in WP-API then it will need changing elsewhere too - a quick glance it seems similar for the post types endpoint