fix issue #23521

Author: aleromano89

app/code/Magento/Downloadable/Test/Unit/Helper/DownloadTest.php

@@ -17,7 +17,10 @@
  */
 class DownloadTest extends \PHPUnit\Framework\TestCase
 {
-    /** @var DownloadHelper */
+	/** @var array Result of get_headers() function */
+	public static $headers;
+
+	/** @var DownloadHelper */
     protected $_helper;
 
     /** @var Filesystem|\PHPUnit_Framework_MockObject_MockObject */
@@ -230,6 +233,7 @@ protected function _setupUrlMocks($size = self::FILE_SIZE, $url = self::URL, $ad
             $this->returnValue($this->_handleMock)
         );
 
+		self::$headers = ['200 OK'];
         $this->_helper->setResource($url, DownloadHelper::LINK_TYPE_URL);
     }
 

app/code/Magento/Downloadable/Test/Unit/_files/download_mock.php

@@ -22,3 +22,13 @@ function mime_content_type()
 {
     return DownloadTest::$mimeContentType;
 }
+
+/**
+ * Override standard function
+ *
+ * @return array
+ */
+function get_headers()
+{
+	return DownloadTest::$headers;
+}