Cancelling an autorenew membership - send notification to authorize.net should default to 'YES"

When creating an autorenew membership for Authorize.net you are sent an email with this in it

This membership will be renewed automatically. You can cancel the auto-renewal option by visiting this web page: http://civi45/civicrm/contribute/unsubscribe?reset=1&mid=35.
You can update billing details for this automatically renewed membership by <a href="http://civi45/civicrm/contribute/updatebilling?reset=1&mid=35">visiting this web page</a>.

On going to the url you are presented the option of whether you want to cancel with A.net. For self-service this option should probably always be on & invisible.

However, as a minimum fix I am fixing the default to 'Yes'