[HttpKernel] instantiate valid commands only

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | debatable ..
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

This skips the default behavior to instantiate `Command` that cannot simply be instantiated. The respective commands have to be registered by other means.