Edited Issue: HtmlHelper.AttributeEncode doesn't support IHtmlString [1202]

There's an overload for HtmlHelper.AttributeEncode(object) that converts the input to a string, and then calls HtmlAttributeEncode -- this prevents correct processing of a pre-encoded object that implements IHtmlString.

This is likely the case because there's an existing function HttpUtility.HtmlEncode(object), but no corresponding overload HttpUtility.HtmlAttributeEncode(object).