Sending custom headers into PrintNode.

[nikolay-shapoval]

What about sending custom headers into PrintNode (for instance X-Idempotency-Key, see https://www.printnode.com/en/docs/api/curl#create-printjob-idempotency)?
We need an opportunity for sending a custom header data.
I can send a pull-request for that.

[JonoHall]

I know it's 3 years late, but I've just created a pull request for this if anyone else is looking for an answer to the Idempotency key:
#42

Tested and working, so hopefully it gets pulled.