React to aspnet/Razor#240.

- Updated tests to account for new casing
- I chose not to update the MVC TagHelpers to use the new casing. We can revisit this later if we want. Essentially instead of:

```C#
public const string AntiForgeryAttributeName = "asp-anti-forgery";

[HtmlAttributeName(AntiForgeryAttributeName)]
public bool? AntiForgery { get; set; }
```

We could do:

```C#
public bool? AspAntiForgery { get; set; }
```

Looks much cleaner but the difference is the usability in the code base has the unneeded `Asp` prefix; I'm indifferent which is why I left it unchanged.

Reacting to: https://github.com/aspnet/Razor/pull/262