lease_seconds should be optional during unsubscribing

[marten-de-vries]

While testing my implementation with https://websub.rocks/hub/104, after clicking the 'Unsubscribe' button (using a public hub) while 'Waiting for unsubscription to be confirmed' I get the following error:

Error!

The verification request was missing the hub.lease_seconds parameter

The spec has the following to say about lease_seconds (5.3 Hub Verifies Intent of the Subscriber):

hub.lease_seconds
REQUIRED/OPTIONAL. The hub-determined number of seconds that the subscription will stay active before expiring, measured from the time the verification request was made from the hub to the subscriber. Hubs MUST supply this parameter for subscription requests. This parameter MAY be present for unsubscribe requests and MUST be ignored by subscribers during unsubscription.

In short, I think the test suite, acting as a subscriber here, violates the last MUST.

(Moved from w3c/websub#122)