Microsoft.Network/FirewallPolicies does not preserve tag key casing

Example:

```
az resource tag --tags TestCasing=Test2 --ids /subscriptions//resourceGroups//providers/Microsoft.Network/firewallPolicies/
{
[...]
"tags": {
"testCasing": "Test2"
},
"type": "Microsoft.Network/FirewallPolicies"
}

```

Expected behaviour is to return:

```
"tags": {
"TestCasing": "Test2"
},
```

Start by reproducing the example with `az resource tag` against a Microsoft.Network/FirewallPolicies resource and compare the returned tag key casing with the request. Then inspect the repository's relevant API specification entry, if identified, to determine whether this behavior is representable there; done means the cause and an actionable specification change or service-side limitation are established.

Written by the indexing model from the issue text.