Add `BeginRouteForm()` HTML helper (#437)

- refactor `DefaultHtmlGenerator.GenerateForm()` to support new method

- also test `Html.BeginForm()`, which was previously untested (touching on #453)
- 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`

nits:
- correct XML comments
- add a few more code comments
- `DefaultTemplatesUtilities.GetHtmlHelperForViewData()` -> `...GetHtmlHelper()`
  for consistency with other overloads
Note that I'm suggesting being more precise about what HTML will be generated. There's no guarantee that routes roundtrip, hopefully they do... but there's no black-magic here, we're generating a URL in HTML - how that URL is handled by the server *later* isn't part of the contract of `@Html.Form`