The sku.tier doesn't take effect while creating app service plan

After tested, seems sku.tier doesn't take effect while creating app service plan. If sku.tier I specified is incorrect, I assume api should throw error, right? Thanks.

Http request:
```
PUT https://management.azure.com/subscriptions/xx-xx-xx-xx/resourceGroups/acctestRG-appservice-test2/providers/Microsoft.Web/serverfarms/demo-app-service-pan

{
"kind": "Windows",
"location": "eastus",
"properties": {
"isXenon": false
},
"sku": {
"name": "F1",
"tier": "Shared",
"size": "F1",
"capacity": 0
},
"tags": {}
}
```

Http response:
```
{
......
"sku": {
"name": "F1",
"tier": "Free",
"size": "F1",
"family": "F",
"capacity": 0
}
}
```

The issue names no repository file or test. Start by locating the App Service serverfarms API specification and comparing its SKU schema with the shown request and response. Done means the expected handling of an invalid or ignored sku.tier is agreed and the corresponding specification behavior is covered by the project's validation or tests.

Written by the indexing model from the issue text.