[extractor/common] Extract timestamp from Last-Modified header

youtube_dl/extractor/generic.py

@@ -2360,7 +2360,7 @@ class GenericIE(InfoExtractor):
         info_dict = {
             'id': video_id,
             'title': self._generic_title(url),
-            'upload_date': unified_strdate(head_response.headers.get('Last-Modified'))
+            'timestamp': unified_timestamp(head_response.headers.get('Last-Modified'))
         }
 
         # Check for direct link to a video