Decorates another BidiMap to ensure it can't be altered.
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 map passed in is already unmodifiable, it is returned.
Parameters: map - the map to decorate, must not be null an unmodifiable BidiMap throws: IllegalArgumentException - if map is null