Inject controller into assistant controller action

As discussed here:
  - https://github.com/KnpLabs/KnpRadBundle/pull/99/files#L0R29
  - https://github.com/KnpLabs/KnpRadBundle/pull/99/files#L0R31

It's a good practice to do so.

I'd also like to celebrate this 100th PR for the KnpRadBundle!!! :birthday: