Fix no subject error on nested collection

[Pasttaga]

Subject

Nested collection in form not work with more than 2 levels.
This PR fix error 'Admin has no subject'.

I am targeting this branch, because it's probably a bug.

Closes #7532.

Changelog

### Fixed
- Set subject of $associationAdmin when collection is append to childs collections.

[jordisala1991]

@Pasttaga or @VincentLanglet do you think this can be tested?

[Pasttaga]

@Pasttaga or @VincentLanglet do you think this can be tested?

I think one more level can be tested in test Sonata\AdminBundle\Tests\Admin\AdminHelperTest::testAppendFormFieldElementWithCollection() i try it without success for the moment.

[VincentLanglet]

@Pasttaga or @VincentLanglet do you think this can be tested?

It could. But I'm not sure it will be easy.

[jordisala1991]

Thanks @Pasttaga