Update of keystone group fails if we doesn't provide some properties

Create stack with such template:
heat_template_version: 2013-05-23

resources:
  admin_user:
    type: OS::Keystone::Group
    properties:
        name: aaa
        roles:
          - role: abc1
            project: admin
Update with
heat_template_version: 2013-05-23

resources:
  admin_user:
    type: OS::Keystone::Group
    properties:
        name: aaa
        roles:
          - role: abc1
            project: demo

BadRequest: {} does not have enough properties (HTTP 400) (Request-ID: req-1934434b-0e13-4d42-926b-487996e3c3e4)