Bugfix json-marshal collaborators in Ticket

[frairon]

Many thanks for the library!

Here's a PR to fix a small bug when json-marshalling a Ticket.

This happens, e.g. when creating a Ticket in ticket.go:242
which marshals the passed Ticket in zendesk.go:117
The json sent to the server is then something like this:

{
  "ticket": {
    "//": "...",
    "collaborators": {},
    "//": "...",
  }
}

Although it should be "collaborators" : [] because it's clearly a list.
The reason is, that json.Marshal(...) is called on a value of Ticket instead of a pointer, i.e. the function func (c * Collaborators) MarshalJSON() ([]byte, error) is never called.

In production there is no problem because Zendesk tolerates the data.
In our application however we mock Zendesk's http interface for unit testing. In the mock we'd like to json.Unmarshal the received data into a Ticket to send it back to the caller. This fails with json: cannot unmarshal object into Go struct field Ticket.ticket.collaborators of type []interface {}.
I'd like to fix that.

[nukosuke]

@frairon.

omg, it's definitely a bug.
After merge, I'll release v0.9.4 which includes this pr. Please try it.

Thank you for that 👍

[nukosuke]

LGTM

[frairon]

@nukosuke Awesome, thanks!