React to aspnet/Razor#208 changes.
Extend `IHtmlGenerator.GenerateSelect()` to make selected values available

nit: use Linq methods, not query syntax in `DefaultHtmlGenerator`
Provide selected values to `<option/>` tag helpers
- value may remain in the `FormContext` beyond `</select>` end tag but will
  be cleaned up at the `</form>` end tag of the containing `<form/>` element
 - `SelectTagHelper` called prior to helpers for contained `<option/>`s and
   not again later
- adjust mock setups to handle new `GenerateSelect()` call
- add assertions for expected `FormContext.FormData` entry

nit: mention #1468 in a test comment
Add `<option/>` tag helper
- #1423