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
not a "pass through `string`" since the method provides a default ("post").  in addition HTML 5 is quite [clear](http://www.w3.org/TR/html5/forms.html#concept-fs-method) that only two values are allowed for the resulting attribute.