SimpleIdServer: [SCIM] PUT operation on immutable group fails

Setup: all the member attributes of a group are defined as immutable as defined here.

Reproduces by the following actions:

1. POST - create a user
2. POST - create a group with the user in it
3. PUT - on the group with the same members payload as in 2

The response is:

{
    "schemas": [
        "urn:ietf:params:scim:api:messages:2.0:Error"
    ],
    "status": "400",
    "scimType": "mutability",
    "detail": "attribute d6c036b6-51f7-4c92-a45f-d8c747fe2304 is immutable"
}

In RFC-7644 section 3.5.1 -

   immutable  If one or more values are already set for the attribute,
      the input value(s) MUST match, or HTTP status code 400 SHOULD be
      returned with a "scimType" error code of "mutability".  If the
      service provider has no existing values, the new value(s) SHALL be
      applied.

Then, it seems to me that if the members in the PUT payload is the same, we should not get a 400 status code.

This same exception occurs on the following, which also seems to not follow this RFC section.

1. POST - create a user
2. POST - create an empty group
3. PUT - on the group with the user in it (to add the user)