Remove HTML helpers with `IDictionary<string, object>` parameters
- see line 2 of #874
- focus on `TextBox[For]()` and `ValidationSummary()`

related fixes included here:
- `TextArea[For]()` documentation incorrectly indicated their `htmlAttributes` parameters were dictionaries
- handle `htmlAttributes` parameters more consistently; create a dictionary only when necessary
Fix for issue #349
Updating tests using Moq to workaround Roslyn changes

https://roslyn.codeplex.com/workitem/246 affects usage of code with the
latest build of Roslyn with Moq v4.2. The workaround involves ensuring a
closure is created. Updating affected tests to make ToString() calls on
local variables to create these closures.
Remove unused `using`s from 3db0a803062df19e83bdd62b63eba47c016af384 change
- see 3db0a803062df19e83bdd62b63eba47c016af384
Make HTML helper `null` handling consistent
- #874 lines 3, 4, and 6
- correct `Value()` to treat a `null` expression name the same as `string.Empty`
- add missing `[NotNull]` attributes in `EditorExtensions` and for `GenerateIdFromName()`
- consistently pass `null` for default expression names to the helpers
 - for example, from extension methods
- add test cases using `null` for expression name

nits:
- correct summary XML comment for `HtmlHelper` class
- use named parameters and prefer interface (not extension) methods in changed calls
- use `string.Empty` instead of `""` in a few tests