Custom modifiers are broken

[Leemonn]

Module versions (please complete the following information):

• vue: 2.4.2
• vue-i18n: 8.24.5

Reproduction Link
https://stackblitz.com/edit/vue-i18n-6abnwm?file=plugins/i18n.js

To Reproduce
Steps to reproduce the behavior:
ex:

1. Use an integrated modifier like upper or capitalize.
2. See that it works
3. Add a custom modifier
4. Use the custom modifier instead of the integrated one.
5. See that the string is not working.

Expected behavior
If the custom modifier is declared then it should apply the modifier and it should show the string.

[Leemonn]

Bump

[DZ-study]

modifiers: { snakeCase: (str) => str.split(' ').join('-') },
snakeCase转换成snakecase，又可以了。。

[kazupon]

fixed with #1512