Cancellation is atomic

[nickevansuk]

"Customer requested cancellation" is an atomic request: it succeeds or fails entirely, in line with other endpoints. It is also idempotent. This should be clarified in the spec in 8.3.2, and checked with the test suite.

The test suite could:

• To confirm idempotence: Make two identical cancellation requests, check that the responses of both are identical and successful.
• To confirm atomicity: Attempt to cancel an Order with three order items, two (A, B) of which can be cancelled and another (C) which cannot (allowCustomerCancellationFullRefund is false). Book A,B,C. Then attempt to cancel B,C (which should fail, atomically). Then cancel A. The Orders feed should show that only A has been cancelled.

[nickevansuk]

Additionally ensure that something like the following is clearly included in the spec (ref: https://github.com/openactive/open-booking-api/issues/102):

The UX of a Broker would need to change to use language such as "Request Cancellation", as the cancellation cannot be guaranteed.