Private DNS RecordSet: ResourceGroup name has wrong case

While working on a new terraform resource for a private DNS Zone A record ( https://github.com/terraform-providers/terraform-provider-azurerm/pull/3849 ) I stumbled across an issue with upper case resource group names.

### Step to reproduce:

* create a resource group named `tesT`
* create a private DNS zone in this resource group ('testdomain.com')
* create an A record in the private DNS zone. ('test')
* API request: https://management.azure.com/subscriptions//resourceGroups/tesT/providers/Microsoft.Network/privateDnsZones/testdomain.com/A/test.testdomain.com?api-version=2018-09-01

The API response for the A record resource is like:

```
{
"id":"\/subscriptions\/\/resourceGroups\/test\/providers\/Microsoft.Network\/privateDnsZones\/testdomain.com\/A\/test",
"name":"test.testdomain.com",
"type":"Microsoft.Network\/privateDnsZones\/A",
"etag":"bfe9801a-698e-4835-a9b5-8cd31bd3e12c",
"properties":{
"metadata":{

},
"fqdn":"test.mtestdomain.com.",
"ttl":300,
"aRecords":[
{
"ipv4Address":"10.10.10.10"
}
],
"isAutoRegistered":false
}
}
```

The problem is the lower case in the `id` attribute for the resource group `test`

### Expected Behaviour:

Having an id with resource group name `tesT`.
This is the behaviour when creating a recordset for the normal DNS zones.

Start by locating the Azure REST specification entry for private DNS zone A record sets and compare it with the normal DNS zone record-set behavior described in the issue. Reproduce the request using a resource group named `tesT`; done means the returned resource ID preserves that casing, with the specification or related validation updated accordingly.

Written by the indexing model from the issue text.