"x-original-url" does not show the original url after add custom domain

I am not sure if it is the right place to file a bug. I have a http-trigger function whose url is `/api/ssr` and here is my `proxies.json`

```json

{
"$schema": "http://json.schemastore.org/proxies",
"proxies": {
"proxy1": {
"matchCondition": {
"route": "/{entry}"
},
"backendUri": "https://example.azurewebsites.net/api/ssr"
}
}
}

```

The request headers `x-original-url` show `/b.node` when I hit `https://example.azurewebsites.net/b.node` and this behaviour is correct. After adding my custom domain, `x-original-url` will always show `/api/ssr` even with using `https://example.azurewebsites.net/b.node`