Address TODO item with new HasNonDefaultEditFormat

- avoid overriding a datetime format if format was already customized
- primarily affects the default Date and Time editor templates because they
  are often used due to `[DataType]` attributes with "default" edit formats

src/Microsoft.AspNet.Mvc.Core/Rendering/Html/DefaultEditorTemplates.cs

@@ -355,9 +355,7 @@ private static void ApplyRfc3339DateFormattingIfNeeded(IHtmlHelper html, string
 
             var metadata = html.ViewData.ModelMetadata;
             var value = metadata.Model;
-
-            // TODO: add ModelMetadata.HasNonDefaultEditFormat and use here (also return if true)
-            if (html.ViewData.TemplateInfo.FormattedModelValue != value)
+            if (html.ViewData.TemplateInfo.FormattedModelValue != value && metadata.HasNonDefaultEditFormat)
             {
                 return;
             }