Add Equality on MockDocumentReference

[suztomo]

    test('FieldValue.arrayUnion() adds unique items', () async {
      final firestore = MockFirestoreInstance();
      // Empty document before updateData
      await firestore.collection('messages').document(uid).setData({
        'reference array': [
          firestore.collection('messages').document('document1')
        ]
      });

      await firestore.collection('messages').document(uid).updateData({
        'reference array': FieldValue.arrayUnion(
            [firestore.collection('messages').document('document1')]),
      });

      final users = await firestore.collection('messages').getDocuments();
      final snapshot = users.documents.first;

      expect(snapshot['reference array'], [
        firestore.collection('messages').document('document1')
      ]);
    });

This failed:

Expected: [MockDocumentReference:MockDocumentReference]
  Actual: [MockDocumentReference:MockDocumentReference, null]
   Which: was MockDocumentReference:<MockDocumentReference> instead of MockDocumentReference:<MockDocumentReference> at location [0]