In ASP.NET 4 this was as easy as routes.LowercaseUrls = true; in the RegisterRoutes handler for the app.
routes.LowercaseUrls = true;
RegisterRoutes
I cannot find an equivalent in ASP
Found the solution.
In the assembly: Microsoft.AspNet.Routing, and the Microsoft.Extensions.DependencyInjection namespace, you can do this in your ConfigureServices(IServiceCollection services) method:
Microsoft.AspNet.Routing
Microsoft.Extensions.DependencyInjection
ConfigureServices(IServiceCollection services)
services.ConfigureRouting(setupAction => { setupAction.LowercaseUrls = true; });