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
 Here's the relevant snippet from MVC 5:
> When the user submits the form, the request will be processed by the route target.

ambiguity level 6/10

A more accurate description wold be something like:

> Renders a &lt;form&gt; start tag to the response. The URL of the form's action attribute will be generated by the route specified by <paramref ...>.