I got this error when using the Twitter gem, and passing in an array.

My code looks innocent enough - wondering what is causing this?

```
 def twitter_get_users(client, user_names=[])
    copy=Array.new(user_names)
    users = client.users(user_names)
  end

TypeError: can't modify frozen string
    from gems/twitter-1.0.0/lib/twitter/client/utils.rb:10:in `gsub!'
    from /gems/twitter-1.0.0/lib/twitter/client/utils.rb:10:in `clean_screen_name!'
    from /gems/twitter-1.0.0/lib/twitter/client/utils.rb:33:in `merge_users_into_options!'

```