[application-gateway] unable to update url-path-map for rule

# Describe the bug

When I try to update an existing agw rule so that it uses a different url-path-map the change is not applied.

command used : `az network application-gateway rule update --resource-group rg-valo-test --gateway-name agw-valo-test --name vnet-valo-test-rqrt --url-path-map "maintenance-upm"`

output:
```
{
"backendAddressPool": null,
"backendHttpSettings": null,
"etag": "W/\"b110ee35-0a69-41b7-af03-058b8ba91bbc\"",
"httpListener": {
"id": "/subscriptions/b183a344-c7ff-44c3-bd41-abeaa2ec03e5/resourceGroups/rg-valo-test/providers/Microsoft.Network/applicationGateways/agw-valo-test/httpListeners/vnet-valo-test-https-front-lstn",
"resourceGroup": "rg-valo-test"
},
"id": "/subscriptions/b183a344-c7ff-44c3-bd41-abeaa2ec03e5/resourceGroups/rg-valo-test/providers/Microsoft.Network/applicationGateways/agw-valo-test/requestRoutingRules/vnet-valo-test-rqrt",
"name": "vnet-valo-test-rqrt",
"priority": null,
"provisioningState": "Succeeded",
"redirectConfiguration": null,
"resourceGroup": "rg-valo-test",
"rewriteRuleSet": null,
"ruleType": "PathBasedRouting",
"type": "Microsoft.Network/applicationGateways/requestRoutingRules",
"urlPathMap": {
"id": "/subscriptions/b183a344-c7ff-44c3-bd41-abeaa2ec03e5/resourceGroups/rg-valo-test/providers/Microsoft.Network/applicationGateways/agw-valo-test/urlPathMaps/vnet-valo-test-upm",
"resourceGroup": "rg-valo-test"
}
}
```

expected output (`urlPathMap.id` has changed):

```
{
"backendAddressPool": null,
"backendHttpSettings": null,
"etag": "W/\"b110ee35-0a69-41b7-af03-058b8ba91bbc\"",
"httpListener": {
"id": "/subscriptions/b183a344-c7ff-44c3-bd41-abeaa2ec03e5/resourceGroups/rg-valo-test/providers/Microsoft.Network/applicationGateways/agw-valo-test/httpListeners/vnet-valo-test-https-front-lstn",
"resourceGroup": "rg-valo-test"
},
"id": "/subscriptions/b183a344-c7ff-44c3-bd41-abeaa2ec03e5/resourceGroups/rg-valo-test/providers/Microsoft.Network/applicationGateways/agw-valo-test/requestRoutingRules/vnet-valo-test-rqrt",
"name": "vnet-valo-test-rqrt",
"priority": null,
"provisioningState": "Succeeded",
"redirectConfiguration": null,
"resourceGroup": "rg-valo-test",
"rewriteRuleSet": null,
"ruleType": "PathBasedRouting",
"type": "Microsoft.Network/applicationGateways/requestRoutingRules",
"urlPathMap": {
"id": "/subscriptions/b183a344-c7ff-44c3-bd41-abeaa2ec03e5/resourceGroups/rg-valo-test/providers/Microsoft.Network/applicationGateways/agw-valo-test/urlPathMaps/maintenance-upm",
"resourceGroup": "rg-valo-test"
}
}
```

az version: `2.0.80`

When used with the `--debug` flag it seems the PUT request is valid (or at least it is what i would expect it to be), so it might be an issue server-side. The return code is 200 while it doesn't show anything as changed.