Fix #1618, Add asp- prefix to custom attributes of MVC tag helpers

- more details in individual commit descriptions
- one highlight is addition of functional test for MVC tag helper sample
Hmm, do we want to do something if the user specifies both `for` and `asp-for`? Ideally it'll just work because the `.MergeAttributes` wont override the user provided `for`. Butttt that could enable users to shoot themselves in the foot. I'm ok with not doing anything, just thought I'd raise the question.