fix(runtime/dotnet): Correctly map callback results

Use the type data available on the deputy object to determine the
relevant JSII type for the result of callbacks, instead of trying to map
into `PrimitiveType.Any`.

Fixes #290
Fixes aws/aws-cdk#1027

packages/jsii-dotnet-runtime/src/Amazon.JSII.Runtime/CallbackExtensions.cs

@@ -76,7 +76,8 @@ static object InvokeMethod(InvokeRequest request, IReferenceMap referenceMap, ou
                 throw new InvalidOperationException($"Received callback for {deputy.GetType().Name}.{request.Method} getter, but this method does not exist");
             }
 
-            returnType = methodInfo.GetCustomAttribute<JsiiMethodAttribute>()?.Returns;
+            JsiiMethodAttribute attribute = methodInfo.GetCustomAttribute<JsiiMethodAttribute>();
+            returnType = attribute?.Returns;
 
             return methodInfo.Invoke(deputy, request.Arguments);
         }
@@ -92,14 +93,15 @@ static object InvokeGetter(GetRequest request, IReferenceMap referenceMap, out T
                 throw new InvalidOperationException($"Received callback for {deputy.GetType().Name}.{request.Property} getter, but this property does not exist");
             }
 
+            JsiiPropertyAttribute attribute = propertyInfo.GetCustomAttribute<JsiiPropertyAttribute>();
+            returnType = attribute?.Type;
+
             MethodInfo methodInfo = propertyInfo.GetGetMethod();
             if (methodInfo == null)
             {
                 throw new InvalidOperationException($"Received callback for {deputy.GetType().Name}.{request.Property} getter, but this property does not have a getter");
             }
 
-            returnType = methodInfo.GetCustomAttribute<JsiiPropertyAttribute>()?.Type;
-
             return methodInfo.Invoke(deputy, new object[] { });
         }