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
I want to capture its state and make sure it's immutable from the perspective of `IntlRelativeFormat`. We don't have this issue in `IntlMessageFormat` since we use the `locales` array right away in the constructor and then never again.