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
u mean others changing the value? it seems to me that this is really not needed since this component will not change the content of the array.