Change MVC tag helpers to not override tag name

- #1523
- test tag name preservation with all MVC tag helpers
 - `<input/>` tag helper generation of a checkbox wasn't previously tested

See individual commit descriptions for more.

/cc @NTaylorMullen @pranavkm 
I don't think we should remove this here since it's a public extension method. This method is responsible for merging a `TagHelperOutput` and a `TagBuilder`, that involves merging all aspects of them. I know we utilize this `Merge` method in a few places so maybe we want to remove it and replace it with something content specific?  Or we could just leave it and hand code the `Content += ` in our `TagHelper`s.