safe_update support

[haizaar]

Good day,

We have a situation where two processes update Zendesk ticket and quite often step on each other's toes causing lost updates.

After doing some research, I found that ZD API indeed supports cooperative locking mechanism as described here: https://develop.zendesk.com/hc/en-us/articles/360059146153#protecting-against-ticket-update-collisions

It will be really great to have this supported in go-zendesk.
WDYT?

[eggman64]

That's a really good idea :)

[alexei-led]

This PR #106 implements the safe_update