Make sure $LOADED_FEATURES gets restored after create block.

Make sure `$LOADED_FEATURES` gets restored after `create` block.

Previously, a reference to the about-to-be-mutated array was being stored as the backup to restore from at the end of the block.

This resulted in `$LOADED_FEATURES` being permanently cleared, causing some files to be required again later, as @jamesotron and I independently experienced while working on Rubinius-targetting languages.