Fix ReactiveValidationObject<T> concurrency issues (#54)

* Update ReactiveValidationObject.cs

* Don't rely on IsValid observable

* Update tests to comply the new behavior

Author: worldbeater

src/ReactiveUI.Validation.Tests/NotifyDataErrorInfoTests.cs

@@ -113,14 +113,13 @@ public void ShouldFireErrorsChangedEventWhenValidationStateChanges()
             Assert.False(viewModel.ValidationContext.IsValid);
             Assert.Single(viewModel.ValidationContext.Validations);
             Assert.Single(viewModel.GetErrors("Name").Cast<string>());
-            Assert.Null(arguments);
 
             viewModel.Name = "JoJo";
 
             Assert.False(viewModel.HasErrors);
             Assert.Empty(viewModel.GetErrors("Name").Cast<string>());
             Assert.NotNull(arguments);
-            Assert.Equal("Name", arguments.PropertyName);
+            Assert.Equal(string.Empty, arguments.PropertyName);
         }
     }
 }

src/ReactiveUI.Validation/Helpers/ReactiveValidationObject.cs

@@ -38,11 +38,8 @@ protected ReactiveValidationObject(IScheduler scheduler = null)
                 .Select(valid => !valid)
                 .ToProperty(this, x => x.HasErrors, scheduler: scheduler);
 
-            ValidationContext
-                .ValidationStatusChange
-                .CombineLatest(Changed, (_, change) => change.PropertyName)
-                .Where(name => name != nameof(HasErrors))
-                .Select(name => new DataErrorsChangedEventArgs(name))
+            ValidationContext.ValidationStatusChange
+                .Select(validity => new DataErrorsChangedEventArgs(string.Empty))
                 .Subscribe(args => ErrorsChanged?.Invoke(this, args));
         }