Add UpdateAssignee method to allow users to change issue assignee

issue.go

@@ -1088,3 +1088,22 @@ func (s *IssueService) RemoveWatcher(issueID string, userName string) (*Response
 
 	return resp, err
 }
+
+// UpdateAssignee updates the user assigned to work on the given issue
+//
+// JIRA API docs: https://docs.atlassian.com/software/jira/docs/api/REST/7.10.2/#api/2/issue-assign
+func (s *IssueService) UpdateAssignee(issueID string, assignee *User) (*Response, error) {
+	apiEndPoint := fmt.Sprintf("rest/api/2/issue/%s/assignee", issueID)
+
+	req, err := s.client.NewRequest("PUT", apiEndPoint, assignee)
+	if err != nil {
+		return nil, err
+	}
+
+	resp, err := s.client.Do(req, nil)
+	if err != nil {
+		err = NewJiraError(resp, err)
+	}
+
+	return resp, err
+}

issue_test.go

@@ -1308,3 +1308,25 @@ func TestIssueService_GetWatchers(t *testing.T) {
 		t.Error("Expected watcher name fred")
 	}
 }
+
+func TestIssueService_UpdateAssignee(t *testing.T) {
+	setup()
+	defer teardown()
+	testMux.HandleFunc("/rest/api/2/issue/10002/assignee", func(w http.ResponseWriter, r *http.Request) {
+		testMethod(t, r, "PUT")
+		testRequestURL(t, r, "/rest/api/2/issue/10002/assignee")
+
+		w.WriteHeader(http.StatusNoContent)
+	})
+
+	resp, err := testClient.Issue.UpdateAssignee("10002", &User{
+		Name: "test-username",
+	})
+
+	if resp.StatusCode != 204 {
+		t.Error("Expected issue not re-assigned.")
+	}
+	if err != nil {
+		t.Errorf("Error given: %s", err)
+	}
+}