Fixed getting wrong subject in sonata_type_collection.

This is additional fix for wrong subject problem (the first fix did @rande: 013456608134c704edb01ff9487d8a664200af11)

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #1568 #991 maybe #1515
| License       | MIT
| Doc PR        | - 

@vhpoet @camilleb @idhard @caponica  Can you please test the latest SonataAdmin code with this PR if it solves your problem with wrong subject returned by getSubject method in collection?