fixes failing select ui test

Fixes the following failing test:

```sh
Error: Uncaught Error: Uncaught AssertionError: expected 'rgba(0, 0, 0, 0)' to equal 'rgb(0, 0, 0)' (http://ebidel.github.io/polymer-experiments/select-element/tools/test/mocha-htmltest.js:34)
    at global.onerror (http://ebidel.github.io/polymer-experiments/select-element/node_modules/mocha/mocha.js:5622:10)
```