Update testrun's assignee should support both username and userkey

Using the following endpoints, try to update a Test Run assignee. Xray should accept both username and userkey.

Affected endpoints:

• PUT /rest/raven/1.0/api/testrun/{testrunId}
• PUT /rest/raven/2.0/api/testrun/{testrunId}
• PUT /rest/raven/1.0/api/testrun/{testrunId}/assignee
• PUT /rest/raven/2.0/api/testrun/{testrunId}/assignee