- helps w/ #453 since `Html.BeginForm()` wasn't previously tested
- provide a `DefaultTemplatesUtilities.GetHtmlHelper()` overload with an
`IHtmlGenerator` parameter
- update `DefaultTemplateUtilities` to use `DefaultHttpContext`
- stop using a mock for this purpose; provides a non-`null` `Request`
nit:
- `DefaultTemplatesUtilities.GetHtmlHelperForViewData()` -> `...GetHtmlHelper()`
for consistency with other overloads
