Fix select switching to multiple in IE

This one was an actual behavioral bug rather than a bug with the tests; our intention was that the first element from the `defaultValue` array would remain selected but IE seemed to be choosing the last one instead. Now we set the value for uncontrolled components in componentDidUpdate when switching from multiple to non-multiple to ensure that a consistent option gets selected.

Test Plan: Ran the ReactDOMSelect tests in jest, phantomjs, IE10, Chrome, and Firefox. Also tested an uncontrolled select manually to make sure that nothing crazy happened when switching between options.