Setting [Route("/")] blocks MapSpaFallbackRoute()

## Repro
```
yo aspnetcore-spa
(Select "Vue")
dotnet run
```
Navigating to "/counter" in the address bar correctly opens the relevant SPA route.

Now edit in `HomeController.cs`:
```C#
[Route("/")] // Add this line
public IActionResult Index()
{
  return View();
}
```
```
dotnet run
```
Navigating to "/counter" in the address bar no longer works (HTTP 404).