allow reference many for sonata_type_collection

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | check travis
| Fixed tickets | n.a.
| License       | MIT
| Doc PR        | n.a.

or is there a specific reason not to allow refernce many here?