directly return inside the BuilderExtensions.UseRouter method

@rynowak first intention was to return the result of `Use` method as I believe is the right thing because you are potentially given an implementation of `IBuilder` as the return value and that has a chance to be different than the original one. I'm not sure though. The whole purpose of returning an implementation of `IBuilder` could be method chaining.

The second intention was to be consistent across the stack as it seems that other builder extensions returns the return value of `Use` method (e.g. aspnet/mvc's [UseMvc](https://github.com/aspnet/Mvc/blob/ecbc179d76cd16037306afc39438eedf4895a0a5/src/Microsoft.AspNet.Mvc/BuilderExtensions.cs#L34) method).