Allow for empty duration when banning users indefinitely

[meutraa]

https://dev.twitch.tv/docs/api/reference#ban-user

To ban a user indefinitely, don’t include this field.

This is the justification for adding omitempty to the json tag.

To put a user in a timeout, include this field and specify the timeout period, in seconds.
The minimum timeout is 1 second and the maximum is 1,209,600 seconds (2 weeks).

By default the Duration would be 0, and this causes twitch to respond with "The value in the duration field is not valid."

[coveralls]

Pull Request Test Coverage Report for Build 3326058350

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 92.867%

---

Totals
Change from base Build 3283876985:	0.0%
Covered Lines:	1341
Relevant Lines:	1444

---

💛 - Coveralls

[nicklaw5]

👍