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 
we're then encouraging a poor behaviour.  #1523 is about tag helper authors, including us, not having the `TagBuilder` just "win" w.r.t. the tag name.

note tag helper authors, including those that derive from the MVC tag helpers, can directly set `TagHelperOutput.TagName`.  that's a special case used just once in the MVC tag helpers -- when the `<input/>` tag helper needs to return two elements (for a checkbox).

I'm now thinking about simply removing this method and changing the code currently using it to handle `Content` directly.  Thoughts?