MTChannelBasedBroker impl ignores trigger.spec.delivery when creating subscriptions.

[n3wscott]

Describe the bug
trigger.spec.delivery is not propagated to subscriptions when using MTChannelBasedBroker classed Brokers.

Expected behavior
trigger.spec.delivery is used if provided.

To Reproduce
Set trigger delivery specs and see that they are not used.

Knative release version
0.22

[n3wscott]

/assign