Allow unassign user from an issue

[Art4]

This PR should fix #308. A user could be unassign from an issue with this code:

$client->issue->update($issueId, [
    'assigned_to_id' => ''
]);

However there is a bug in Redmine when the REST API is accessed in XML format. See https://www.redmine.org/issues/33744

So this bugfix will only work after the bug in Redmine is fixed. A temporary workaround is posted here: #308 (comment)

[kbsali]

👍