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.
- add more `DefaultTemplatesUtilities.GetHtmlHelper()` overloads, all
returning a strongly-typed helper
- also correct that method to ensure template helpers each get a unique
`IHtmlHelper` instance