Ignore metadata for frozen classes.

[voxik]

This makes test suite pass with Ruby 2.2. Basically, it tries to avoid modification of frozen object due to translation metadata.

However, I am not really sure if this is the best approach. May be the translated object should be always just some kind of facade, since for some cases the metadata won't be available. On the other hand, this is not new error, it is just more exposed due to frozen true/false objects in Ruby 2.2

[radar]

Thanks for this fix :) I was taking a look at this yesterday too and came up with roughly the same code.

return {} if frozen?
@translation_metadata ||= {}

It's not much different, so I'm going to merge your changes as-is.

Thanks for your contribution!