- #524
- add tests of `DropDownList` and `ListBox` HTML helpers
- extend tests of `<select/>` tag helper
- add scenario to a functional test
New `HtmlHelperSelectTest` does not cover everything. In future (see #453):
- use non-`null` `optionLabel` and `htmlAttributes` arguments
- confirm value sources and their priorities
- mock an `IHtmlGenerator` and confirm how it is called
- mock an `IHtmlHelper` and confirm how extension methods call that
