fix: authorize string as key for datasets

[misaert]

Q	A
Bug fix?	yes
New feature?	no

When using dataset with string as array key in projects, PHPStan refuses because of:

$ vendor/bin/pest

   PASS  Tests\Unit\DatasetTest
  ✓ it tests with string key for dataset with data set "with value `true`" (true)

  Tests:  1 passed
  Time:   0.04s

$ vendor/bin/phpstan analyze
 ------ ---------------------------------------------------------------------------------------------------------------------------------------------------------- 
  Line   tests/Unit\DatasetTest.php                                                                                                                
 ------ ---------------------------------------------------------------------------------------------------------------------------------------------------------- 
  12    Parameter #1 $data of method Pest\PendingObjects\TestCall::with() expects Closure|iterable<int, mixed>|string, array('with value `true`' => true) given.  
 ------ ---------------------------------------------------------------------------------------------------------------------------------------------------------- 

                                                                                                                        
 [ERROR] Found 1 error

for the test below:

it('tests with string key for dataset', function (bool $expected) {
    assertTrue($expected);
})->with([
    'with value `true`' => true
]);

This PR fixes this.