Make sure Observable method is declared progressive

As discussed in the review, this extra declaration is a requirement to
make sure that people really want the method to be progressive and not
return a IObservable derived type by accident.

Code-Sharp#238

src/netstandard/WampSharp/WAMP2/V2/Api/CalleeProxy/CalleeProxyInterceptorFactory.cs

@@ -31,6 +31,8 @@ private static Type GetRelevantInterceptorType(MethodInfo method)
 
             if (returnType.IsGenericType && returnType.GetGenericTypeDefinition() == typeof(IObservable<>))
             {
+                MethodInfoValidation.ValidateProgressiveObservableMethod(method);
+
                 genericArgument = returnType.GetGenericArguments()[0];
                 interceptorType = typeof(ObservableCalleeProxyInterceptor<>);
             }

src/netstandard/WampSharp/WAMP2/V2/Rpc/Callee/Reflection/MethodInfoValidation.cs

@@ -32,7 +32,7 @@ public static void ValidateTupleReturnType(MethodInfo method)
 
                     IList<string> transformNames = attribute.TransformNames;
 
-                    List<string> tupleNames = 
+                    List<string> tupleNames =
                         transformNames.Take(tupleLength).ToList();
 
                     ValidateTupleReturnType(method, tupleNames);
@@ -47,7 +47,7 @@ private static void ValidateTupleReturnTypeWithOutRefParameters(MethodInfo metho
                 method.GetParameters().Where(x => x.IsOut || x.ParameterType.IsByRef)
                   .Select(x => x.Name);
 
-            ICollection<string> intersection = 
+            ICollection<string> intersection =
                 tupleNames.Intersect(outOrRefNames).ToList();
 
             if (intersection.Count > 0)
@@ -74,6 +74,14 @@ public static void ValidateSyncMethod(MethodInfo method)
             ValidateTupleReturnType(method);
         }
 
+        internal static void ValidateProgressiveObservableMethod(MethodInfo method)
+        {
+            if (!method.IsDefined(typeof(WampProgressiveResultProcedureAttribute)))
+            {
+                ThrowHelper.ObservableMethodNotDeclaredProgressive(method);
+            }
+        }
+
         public static void ValidateAsyncMethod(MethodInfo method)
         {
             ParameterInfo[] parameters = method.GetParameters();
@@ -162,6 +170,12 @@ public static void ProgressiveParameterTypeMismatch(MethodInfo method, Type retu
                     ($"Method {method.Name} of type {method.DeclaringType.FullName} is declared as a progressive WAMP procedure, but its last (or second to last) parameter is not a IProgress of its return type. Expected: IProgress<{returnType.FullName}>");
             }
 
+            public static void ObservableMethodNotDeclaredProgressive(MethodInfo method)
+            {
+                throw new ArgumentException
+                    ($"Method {method.Name} of type {method.DeclaringType.FullName} is returning an IObservable and therefore is required to be declared as a progressive WAMP procedure, but it is not. Please use the [WampProgressiveResultProcedure] attribute.");
+            }
+
             public static void ProgressiveParameterTupleMismatch(MethodInfo method)
             {
                 throw new ArgumentException
@@ -199,4 +213,4 @@ public static void CancellationTokenMustBeLastParameter(MethodInfo method)
             }
         }
     }
-}
+}

src/netstandard/WampSharp/WAMP2/V2/Rpc/Callee/Reflection/OperationExtractor.cs

@@ -59,6 +59,7 @@ protected IWampRpcOperation CreateRpcMethod(Func<object> instanceProvider, ICall
 
             if (method.ReturnType.IsGenericType && method.ReturnType.GetGenericTypeDefinition() == typeof(IObservable<>))
             {
+                MethodInfoValidation.ValidateProgressiveObservableMethod(method);
                 return CreateProgressiveObservableOperation(instanceProvider, method, procedureUri);
             }
             else if (!typeof (Task).IsAssignableFrom(method.ReturnType))