Decorates another Bag to ensure it can't be altered.
This class is Serializable from Commons Collections 3.1.
since: Commons Collections 3.0 version: $Revision: 155406 $ $Date: 2005-02-26 12:55:26 +0000 (Sat, 26 Feb 2005) $ author: Stephen Colebourne
If the bag passed in is already unmodifiable, it is returned.
Parameters: bag - the bag to decorate, must not be null an unmodifiable Bag throws: IllegalArgumentException - if bag is null