Model plural label should be called with a Numeric, not a String

Hi,

I'm using _rails-i18n_ to have some translations for free in my rails app. The issue is that the common rule for translations is based on the `count` argument like this:
(`n` being the value of `count`)

``` ruby
lambda { |n| n >= 0 && n < 2 ? :one : :other }
```

I get an error in this block because of the #1517 PR that uses `"other"` to get the `:other` key in translation.

I propose to use `Float::INFINITY` instead. It's a Numeric and it would keep the correct behavior without risking this kind of issue.