Correct initialization of `IdAttributeDotReplacement

- #704 part 1 of 2
- doesn't help tag builders unless done in `DefaultHtmlGenerator`

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

@@ -39,6 +39,9 @@ namespace Microsoft.AspNet.Mvc.Rendering
             _antiForgery = antiForgery;
             _metadataProvider = metadataProvider;
             _urlHelper = urlHelper;
+
+            // Underscores are fine characters in id's.
+            IdAttributeDotReplacement = "_";
         }
 
         /// <inheritdoc />

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

@@ -42,9 +42,6 @@ namespace Microsoft.AspNet.Mvc.Rendering
             _viewEngine = viewEngine;
             _htmlGenerator = htmlGenerator;
             MetadataProvider = metadataProvider;
-
-            // Underscores are fine characters in id's.
-            IdAttributeDotReplacement = "_";
         }
 
         /// <inheritdoc />