[extractor/common] fallback on utf-8 when charset is not found

fixes #2721

youtube_dl/extractor/common.py

@@ -251,7 +251,10 @@ class InfoExtractor(object):
             with open(filename, 'wb') as outf:
                 outf.write(webpage_bytes)
 
+        try:
             content = webpage_bytes.decode(encoding, 'replace')
+        except LookupError:
+            content = webpage_bytes.decode('utf-8', 'replace')
 
         if (u'<title>Access to this site is blocked</title>' in content and
                 u'Websense' in content[:512]):