[youtube:channel] Fix the extraction of autogenerated channels

The ajax pages are empty, now it looks directly in the channel's /videos page

youtube_dl/extractor/youtube.py

@@ -1594,20 +1594,31 @@ class YoutubeChannelIE(InfoExtractor):
         # Download channel page
         channel_id = mobj.group(1)
         video_ids = []
+        url = 'https://www.youtube.com/channel/%s/videos' % channel_id
+        channel_page = self._download_webpage(url, channel_id)
+        if re.search(r'channel-header-autogenerated-label', channel_page) is not None:
+            autogenerated = True
+        else:
+            autogenerated = False
 
-        # Download all channel pages using the json-based channel_ajax query
-        for pagenum in itertools.count(1):
-            url = self._MORE_PAGES_URL % (pagenum, channel_id)
-            page = self._download_webpage(url, channel_id,
-                                          u'Downloading page #%s' % pagenum)
-
-            page = json.loads(page)
-
-            ids_in_page = self.extract_videos_from_page(page['content_html'])
-            video_ids.extend(ids_in_page)
-
-            if self._MORE_PAGES_INDICATOR not in page['load_more_widget_html']:
-                break
+        if autogenerated:
+            # The videos are contained in a single page
+            # the ajax pages can't be used, they are empty
+            video_ids = self.extract_videos_from_page(channel_page)
+        else:
+            # Download all channel pages using the json-based channel_ajax query
+            for pagenum in itertools.count(1):
+                url = self._MORE_PAGES_URL % (pagenum, channel_id)
+                page = self._download_webpage(url, channel_id,
+                                              u'Downloading page #%s' % pagenum)
+    
+                page = json.loads(page)
+    
+                ids_in_page = self.extract_videos_from_page(page['content_html'])
+                video_ids.extend(ids_in_page)
+    
+                if self._MORE_PAGES_INDICATOR not in page['load_more_widget_html']:
+                    break
 
         self._downloader.to_screen(u'[youtube] Channel %s: Found %i videos' % (channel_id, len(video_ids)))