Fix fallback backend so that it works for an array of keys

[IrvinFan]

When multiple keys are passed to translate() as an array the following code in i18n.rb will loop through the keys to apply the fallback translation on each one of them. It re-uses the same options while doing so. As a result it only works for the first key because subsequent translation fails due to polluted options[:fallback].

        if key.is_a?(Array)
          key.map { |k| backend.translate(locale, k, options) }
        else
          backend.translate(locale, key, options)
        end

This PR fixes the issue by creating a new options object when calling super so that original options is not modified and can be re-used.

[radar]

I believe this was fixed in #304. Thanks anyway @IrvinFan!