ensure Globalize.with_locale resets the locale to its previous value

If the block given to <tt>Globalize.with_locale</tt> raises an exception the locale is not reset:

```
> puts Globalize.locale
en
> Globalize.with_locale(:ja) { puts Globalize.locale }
ja
> puts Globalize.locale
en

> Globalize.with_locale(:ja) { puts Globalize.locale; raise "WTF!" }
ja
RuntimeError: WTF!
> puts Globalize.locale
ja
```