Falling through to IIS now requires opt-in

[Tratcher]

A change has been made to the IIS ASP.NET 5 server (Microsoft.AspNet.Server.IIS) such that applications must opt into IIS fallback. Previously when a request reached the end of the managed pipeline without being handled it would automatically unwind the managed pipeline and fall back to IIS. Now the end of the managed pipeline will return an empty 404 response like on other servers. To opt into the fall-through use the new extension app.RunIISPipeline(); at the end of your managed pipeline. This extension will no-op on non-IIS servers.

The primary impact is for static files. Before there was significant confusion on if requests were being handled by the managed static file ASP.NET middleware or the native static file IIS module. Now IIS will not try to serve static files without the opt-in extension. Alternatively applications can use the app.UseStaticFiles() or app.UseFileServer() extensions to use the managed static file middleware.

See aspnet/Hosting#323 for discussion.