[5.8] Convert percentage sign in filename fallback

[barryvdh]

This is a follow-up to #28551, and similar to #6097 and how it's done in the current ResponseFactory:

framework/src/Illuminate/Routing/ResponseFactory.php

Lines 159 to 179 in 7ac7818

	public function download($file, $name = null, array $headers = [], $disposition = 'attachment')
	{
	$response = new BinaryFileResponse($file, 200, $headers, true, $disposition);
	if (! is_null($name)) {
	return $response->setContentDisposition($disposition, $name, $this->fallbackName($name));
	}
	return $response;
	}
	/**
	* Convert the string to ASCII characters that are equivalent to the given name.
	*
	* @param string $name
	* @return string
	*/
	protected function fallbackName($name)
	{
	return str_replace('%', '', Str::ascii($name));
	}

The fallback can't include a percentage sign, so this check is added.