Propagate original `locales` value to MessageFormat

This fixes an issue where the original `locales` value the user passed to the `IntlRelativeFormat` constructor was not being propagated to the internal `IntlMessageFormat` instances. Instead, only the resolved root locale was being propagated.

Fixes #17
```js
var locales = ['en-US', 'en'];
var rf = new IntlRelativeFormat(locales);
locales.unshift('fr');
rf.format(Date.now());
```