fix(src/index.js): fallbackRoot should have higher priority than formatFallbackMessages (#779)

[masongzhi]

fix #779
when formatFallbackMessages is true, _render in _warnDefault should not limited to string

[exoego]

Thanks for opening PR.
Could you read and follow the below pull request guidelines ? (dist should not be changed).
https://github.com/kazupon/vue-i18n/blob/v8.x/CONTRIBUTING.md#pull-request-guidelines

[masongzhi]

Thanks for opening PR.
Could you read and follow the below pull request guidelines ? (dist should not be changed).
https://github.com/kazupon/vue-i18n/blob/v8.x/CONTRIBUTING.md#pull-request-guidelines

Thanks, now I restore the dist directory, and rebase from kazupon/vue-i18n that test run successfully

[exoego]

LGTM

[kazupon]

Good job! Thanks!