Defining charset to "utf8".

TNL-6553

(cherry-picked from f1655051)

common/lib/xmodule/xmodule/video_module/video_handlers.py

@@ -269,7 +269,8 @@ class VideoStudentViewHandlers(object):
                     headerlist=[
                         ('Content-Disposition', 'attachment; filename="{}"'.format(transcript_filename.encode('utf8'))),
                         ('Content-Language', self.transcript_language),
-                    ]
+                    ],
+                    charset='utf8'
                 )
                 response.content_type = transcript_mime_type