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
Add unit test to check that locales is propagated
Merge pull request #18 from ericf/propagate-locales

Propagate original `locales` value to MessageFormat