MVC tag helpers should not override tag name in Razor source
- #1523
- remove `TagHelperOutput.Merge()` extension method entirely
- test tag name preservation with all MVC tag helpers
 - `<input/>` tag helper generation of a checkbox wasn't previously tested

nits:
- fix argument order in a couple of `Assert.Equal()` calls
- remove use of "original tag name"
Address small `InputTagHelper` bug discovered while testing
- Type attribute value was "Type", not "type", when determined from template hints

nit: remove useless condition
Add more tests of `<input/>` tag helper
- scenarios where helper takes different code paths were not well-explored

nits:
- improve a few comments and names
- refactor some setup code into helper methods
- use `EmptyModelMetadataProvider`, not `DataAnnotationsModelMetadataProvider`

Test gap around `GenerateTextBox()`'s `format` argument is temporary.