Add `BeginRouteForm()` HTML helper
- refactor `DefaultHtmlGenerator.GenerateForm()` to support new method

nits:
- correct XML comments
- add a few more code comments
Add `Html.BeginForm()` and `Html.BeginRouteForm()` tests
- 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
Address PR comments in product code
- XML comment improvements
Address PR comments in test code
- add missing license headers
- make "post" more obvious
- use `Assert.IsAssignableFrom()`

nit: remove unused `using`s in `HtmlHelperLinkGenerationTest`
Per PR discussion
- clean up "the the" in XML comments
- simplify refactoring VS did when I renamed `GetHtmlHelperForViewData()`
 - fix existing issue in `HtmlHelperCheckboxTest.CheckBoxReplacesUnderscoresInHtmlAttributesWithDashes()`
   (was using a `HtmlHelper<ViewDataDictionary<TestModel>>`)