When updating fields such as Assignee the current updateIssueField method in RESTClient.java doesn't work. For the assignee field you need the JSON to be in the following format when using fields:

"fields": {
"assignee":

}

The current code assumes that the field will always be a string. Ideally I would like to not have to put JSON in for the field value but that would at least work. I'm not sure if assignee is the only field that takes a different format than just a string value. Using the old soap version of this plugin did work, so I am sure this is a new situation with the REST API. An update to this would be greatly appreciated. Either make the field box more robust or have a separate option for the assignee or other fields in this situation would work as well. Thanks!