Updated HTML Validator Helpers to Render Valid HTML Code

Making a pull request to fix a minor html rendering bug in the `ValidatorInternal` method of the `HtmlHelpers` class. This pull request changes the `TagBuilder` used by the ValidatorInternal method to use `TagRenderMode.SelfClosing` when rendering out the final HTML code.

The current implementation renders img tags with a closing img tag which per the [HTML5 spec](http://dev.w3.org/html5/markup/img.html) is invalid as the img tag is considered a void element. While not that big of an issue, it will keep IE from spitting out unmatched end tag warnings to the console.