update aks encounter error when using user assigned identity

swagger link: https://github.com/Azure/azure-rest-api-specs/blob/HEAD/specification/containerservice/resource-manager/Microsoft.ContainerService/stable/2020-12-01/managedClusters.json#L416

reproduce step:
1. create a cluster with user assigned identity
2. get the cluster and change some property
3. update

the error message:
```Invalid value for the identities '/subscriptions/sub-id/resourceGroups/resource-group-name/providers/Microsoft.ManagedIdentity/userAssignedIdentities/identity-name'. The 'UserAssignedIdentities' property keys should only be empty json objects, null or the resource exisiting property."```

after investigation, the reason is:
`identity.UserAssignedIdentities` is a map, the value is readonly struct. if the update rest api contains the readonly infomation, the backend will throw this error.

some times before, there was no such limitation.

Have checked with AKS team, they says there is no such error in their codebase.
needs check from ARM team

Start with specification/containerservice/resource-manager/Microsoft.ContainerService/stable/2020-12-01/managedClusters.json at line 416, then reproduce the user-assigned-identity update described in the issue. Determine whether the REST specification represents readonly UserAssignedIdentities values in the update request; done means the update succeeds without the reported ARM validation error.

Written by the indexing model from the issue text.