Format problem details (#36)

* problem details formatting

* Moniker prep for Problem Details

fundamentals/middleware/problem-details-service/Program.cs

@@ -42,7 +42,7 @@ app.UseHttpsRedirection();
 app.Use(async (context, next) =>
 {
     var mathErrorFeature = new MathErrorFeature();
-    context.Features.Set(mathErrorFeature);
+    context.Features.Set(new MathErrorFeature());
     await next(context);
 });