[HttpFoundation] Remove body-related headers when sending the response, if body is empty

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

I've updated the implementation for informational and 204 or 304 responses. They will now, as they have no content, not return headers like `content-type` or `content-length`.

I'm unsure about `content-length` - we could also set it hardcoded to zero ... but I thought, that (because the specs say that it just can't have a response-body) the system should not return anything here.