V0.5.3 interpolation fix

This is a stab at a fix for this issue which I believe is limited to the 0.5.x version of this gem.

https://github.com/svenfuchs/i18n/issues/228

In summary having :rescue_format set to :html in the translate method when a default arg was present was incorrectly triggering the interpolation logic to fire off. In my Rails app this caused interpolation errors when a default option contained the % character.

The same test passes without any code modification needed in master thus I'm only proposing this change to the 0.5.0 branch.