fails:Array#keep_if returns the same array if no changes were made
fails:Array#keep_if deletes elements for which the block returns a false value
fails:Array#keep_if returns an enumerator if no block is given
fails:Array#keep_if on frozen objects returns an Enumerator if no block is given
fails:Array#keep_if on frozen objects with truthy block raises a RuntimeError
fails:Array#keep_if on frozen objects with falsy block raises a RuntimeError
