Idempotency when creating a ticket

[huer12]

We need the option to pass the idempotency key in the header when creating a ticket. See https://developer.zendesk.com/api-reference/ticketing/introduction/#idempotency

I created a PR for it. https://github.com/Speedygeek/ZendeskApi_v2/pull/596 Could you have a look at it, if that proposal works for you? Thanks!