Az.Functions Update FunctionApp that has system and user-assigned managed identity

## Description
If a Function App has both, a `SystemAssigned` and `UserAssigned` identity, `Update-FunctionApp` will fail, regardless of what's being updated.

## Steps to reproduce

Azure Portal let's you configure both managed identity types at the same time. The configuration is shown in the resource JSON as

```JSON
"identity": {
"type": "SystemAssigned, UserAssigned",
[...]
}
```

Once you have this configuration, run the following.

```powershell
Update-AzFunctionApp -ResourceGroupName -Name -Tag @{'test'='test'}
```

## Environment data

```

```

## Module versions

```powershell
get-module -name Az.Functions,Az.Accounts | fl name, version

Name : Az.Accounts
Version : 2.5.1

Name : Az.Functions
Version : 3.1.0
```

## Error output

```
Message : Unknown IdentityType 'SystemAssigned, UserAssigned'
StackTrace :
Exception : System.InvalidOperationException
InvocationInfo : {}
Line : throw $errorRecord

Position : At C:\Users\...\Documents\PowerShell\Modules\Az.Functions\3.1.0\custom\HelperFunctions.ps1:807 char:5
+ throw $errorRecord
+ ~~~~~~~~~~~~~~~~~~
```