Autorest generated cmdlet throwing "Argument passed in is not serializable" error

This is for AzureStack specific admin modules
Some of the generated cmdlets throw "Argument passed in is not serializable" error when incorrect parameter value is passed. In the below examples, "TargetSubscriptionId" parameter value is correct for the first one but not for the second one.
```
PS C:\Windows\system32> Get-AzsUserSubscription -TargetSubscriptionId "966bbf24-21ce-43bd-8735-e1a4c0eed411"

DelegatedProviderSubscriptionId DisplayName ExternalReferenceId OfferId
------------------------------- ----------- ------------------- -------
209523fa-09a8-4642-93df-58496efe1ca2 EventRPBVTTenantSubscription /subscriptions/209523fa-09a8-4642-93df-58496efe1ca2/resourceGroups/EventRPBVTTe...

PS C:\Windows\system32> Get-AzsUserSubscription -TargetSubscriptionId "966bbf24-21ce-43bd-8735-e1a4c0eed418"
Get-AzsUserSubscription : Argument passed in is not serializable.
Parameter name: value
At line:1 char:1
+ Get-AzsUserSubscription -TargetSubscriptionId "966bbf24-21ce-43bd-873 ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : NotSpecified: (:) [Get-AzsUserSubscription_Get], ArgumentException
+ FullyQualifiedErrorId : Microsoft.Azure.PowerShell.Cmdlets.SubscriptionsAdmin.Cmdlets.GetAzsUserSubscription_Get
```
details here: https://github.com/Azure/autorest.powershell/issues/755