[TypeDeclarationDocblocks] Use unique type when all types equals on AddReturnDocblockForCommonObjectDenominatorRector

[samsonasik]

Given the folllowing code:

final class UniqueTypeSingle
{
    public function getExtensions(): array
    {
        return [
            new \DateTime('now'),
            new \DateTime('tomorrow'),
        ];
    }
}

It got diff:

+    /**
+     * @return \DateTimeInterface[]
+     */
     public function getExtensions(): array
     {
        return [
            new \DateTime('now'),
            new \DateTime('tomorrow'),
        ];
    }

which should use \DateTime only, as all unique type is single. On complex application, it can cause PHPStan notice that not instanceof type

  - '#Parameter \#1 \$abc of method XYZ expects \DateTimeInterface, \DateTime given#'
  🪪 argument.type

[samsonasik]

Fixed 🎉 /cc @TomasVotruba

[samsonasik]

All checks have passed 🎉 @TomasVotruba I am merging it ;)