Nested groups should apply to hashes too

[Uplink03]

Q	A
Bug report?	maybe
Feature request?	maybe
BC Break report?	no
RFC?	no

Steps required to reproduce the problem

$x = [
    'a' => [objects],
    'b' => [objects],
];

$context = SerializerContext::create()->setGroups([
    'group1',
    'b' => [
        'group2',
    ],
]);

Expected Result

• When serializing the hash, I expected to have 'group2' apply to the objects under the 'b' key.

Actual Result

• The nested group for 'b' has no effect.

As far as I can tell from browsing the source code, there's no easy fix. Hashes would have to be visited in a similar manner as to objects, which currently isn't the case.

I tried to work around with:

• an anonymous class (this crashes hard in some ORM code)
• stdClass with dynamic members (this resulted in an empty JSON object)

For now I resorted to creating a class with all the required members defined on it, but it's a bit of overkill for this use case.