add |safe to shipping_method.description, as it done in shipping_methods...

....html

shipping_method.description can contain html tags. In [shipping_methods.html](https://github.com/tangentlabs/django-oscar/blob/master/oscar/templates/oscar/checkout/shipping_methods.html#L36) it is respected, and in [checkout.html](https://github.com/tangentlabs/django-oscar/blob/master/oscar/templates/oscar/checkout/checkout.html#L63) it is not.