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 
are you objecting to `// ... can merge the TagBuilder in directly`?  what would you change?

this comment contrasts the situation with the special case a bit later in the file.  and "directly" is certainly correct when compared with the checkbox case.