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.