Update interface

Author: Sergei Lomakov

CHANGELOG.md

@@ -33,6 +33,7 @@
 - Images in Layouts other than first Master Slide within Template file causes corrupted PPTX - @maartenba CP-4596
 - Fixed A3 and A4 formats dimensions - @delphiki GH-16
 - Fixed custom document layout - @delphiki GH-18
+- Filename parameter is required for IWriter::save method
 
 ### Miscellaneous
 

Classes/PHPPowerPoint/Writer/IWriter.php

@@ -40,5 +40,5 @@ interface PHPPowerPoint_Writer_IWriter
      * @param  string    $pFilename
      * @throws Exception
      */
-    public function save($pFilename = null);
+    public function save($pFilename);
 }