Fix freeform invite in document.invite

[holdMyTea]

Fixing the TypeError: inviteData.to.forEach is not a function for the similar calls:

api.document.invite({
  token: 'your auth token',
  id: 'document id',
  data: {
    from: 'email address',
    to: 'email address',
  },
}, (err, res) => {
  // handle error or process response data
});

[holdMyTea]

I can't imagine, I've missed anniversary of this PR, but still:

Happy Birthday!!1 🎉 🎉 🎉

[rtereshchenko]

@holdMyTea Maybe some day it will be merged)