[HttpKernel] HttpRenderer fix + enhancements

| Q             | A
| ------------- | ---
| Bug fix?      | yes, 2nd commit
| New feature?  | yes, interfaces
| BC breaks?    | yes, but this is could be considered as a bug fix (`private` -> `protected`) 
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | User facing API is the same