Back `HtmlHelper.Html5DateRenderingMode` with new `ViewContext` property

- provides expected scoped semantics
- helps a bit with #448 but does not provide an app-wide default

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

@@ -59,7 +59,17 @@ namespace Microsoft.AspNet.Mvc.Rendering
         }
 
         /// <inheritdoc />
-        public Html5DateRenderingMode Html5DateRenderingMode { get; set; }
+        public Html5DateRenderingMode Html5DateRenderingMode
+        {
+            get
+            {
+                return ViewContext.Html5DateRenderingMode;
+            }
+            set
+            {
+                ViewContext.Html5DateRenderingMode = value;
+            }
+        }
 
         /// <inheritdoc />
         public string IdAttributeDotReplacement { get; set; }

src/Microsoft.AspNet.Mvc.Core/ViewContext.cs

@@ -41,6 +41,7 @@ namespace Microsoft.AspNet.Mvc
         {
             _formContext = viewContext.FormContext;
             ClientValidationEnabled = viewContext.ClientValidationEnabled;
+            Html5DateRenderingMode = viewContext.Html5DateRenderingMode;
             ValidationSummaryMessageElement = viewContext.ValidationSummaryMessageElement;
             ValidationMessageElement = viewContext.ValidationMessageElement;
 
@@ -64,6 +65,14 @@ namespace Microsoft.AspNet.Mvc
 
         public bool ClientValidationEnabled { get; set; }
 
+        /// <summary>
+        /// Set this property to <see cref="Html5DateRenderingMode.Rfc3339" /> to have templated helpers such as
+        /// <see cref="IHtmlHelper.Editor" /> and <see cref="IHtmlHelper{TModel}.EditorFor" /> render date and time
+        /// values as RFC 3339 compliant strings. By default these helpers render dates and times using the current
+        /// culture.
+        /// </summary>
+        public Html5DateRenderingMode Html5DateRenderingMode { get; set; }
+
         /// <summary>
         /// Element name used to wrap a top-level message generated by <see cref="IHtmlHelper.ValidationSummary"/> and
         /// other overloads.