Fixes xamarin-bug #8965: AllowHtmlAttribute ignored throwing exception on POST using ASP.NET mvc3

Mono's implementation of Microsoft.Web.Infrastructure was (incorrectly)
applying validation over 'unvalidated collections' cause HttpRequest's
internal collection is being replaced by a new LazyWebROCollection
which internally performs validation inside it's Get() methods.

However, ValidationUtility.GetUnvalidatedCollections(..) should not
perform validation, as this is what MVC (when using AllowHtmlAttribute)
expects.

See: https://bugzilla.xamarin.com/show_bug.cgi?id=8965
License: This patch is under MIT/X11 license.