Fix error when you have wrapped defaults in Rails 4

Hi guys,

We're using Localeapp in a Rails 4 app and we're having a problem when we use defaults, because Rails wraps the strings inside an array and Localeapp gem is simply ignoring arrays. This looks wrong. I believe Localeapp should consider arrays when they have a valid text inside and send the key for translations with that text as the default. So heres a PR for that. If you can merge it ASAP we will appreciate so we don't need to point to my repo in our project.