Fix Cube.intersection() for descending bounds.

This fixes an bug in Cube.intersection() where it fails to operate on a monotonically descending bounded coordinate.

For background, see this [Google groups posting](https://groups.google.com/d/msg/scitools-iris/jq6mEn32QQo/unzl3MYS8bgJ) from pbentley and his corresponding [gist](https://gist.github.com/rockdoc/9cb41bbf85fed08fe42b).