Fix fallback backend so that it works for an array key

[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. The current code 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.

[IrvinFan]

Close this as the official PR is here: ruby-i18n#321