Laravel Vapor Support: downloadExcel Eloquent Mixin

[agraaff-inhance]

Right now the downloadExcel Eloquent mixin function does not support adding the necessary headers to allow Excel file downloads when running on Vapor. The required header is "X-Vapor-Base64-Encode: true". I have Laravel Excel does support adding headers to responses but does not surface it throughh the Mixin. Would it be okay to submit a PR to add a 'headers' variable to the downloadExcel callback function definition in DownloadCollection.php and then adding it to the $export->download() call in the last line of the callback function.

public function downloadExcel()
    {
        return function (string $fileName, string $writerType = null, bool $withHeadings = false, ### **array $headers = []**) {
            $export = new class($this, $withHeadings) implements FromCollection, WithHeadings
            {
                use Exportable;

                /**
                 * @var bool
                 */
                private $withHeadings;

                /**
                 * @var Collection
                 */
                private $collection;

                /**
                 * @param  Collection  $collection
                 * @param  bool  $withHeading
                 */
                public function __construct(Collection $collection, bool $withHeading = false)
                {
                    $this->collection   = $collection->toBase();
                    $this->withHeadings = $withHeading;
                }

                /**
                 * @return Collection
                 */
                public function collection()
                {
                    return $this->collection;
                }

                /**
                 * @return array
                 */
                public function headings(): array
                {
                    if (!$this->withHeadings) {
                        return [];
                    }

                    $firstRow = $this->collection->first();

                    if ($firstRow instanceof Arrayable || \is_object($firstRow)) {
                        return array_keys(Sheet::mapArraybleRow($firstRow));
                    }

                    return $this->collection->collapse()->keys()->all();
                }
            };

            return $export->download($fileName, $writerType, ### **$headers**);
        };
    } 

[patrickbrouwers]

@agraaff-inhance I'm fine with that PR yes