[utils] Default age_limit to None

If we can't parse it, it means we don't have any information, not that the content is unrestricted.

youtube_dl/extractor/common.py

@@ -138,6 +138,8 @@ class InfoExtractor(object):
 
     Unless mentioned otherwise, the fields should be Unicode strings.
 
+    Unless mentioned otherwise, None is equivalent to absence of information.
+
     Subclasses of this one should re-define the _real_initialize() and
     _real_extract() methods and define a _VALID_URL regexp.
     Probably, they should also be added to the list of extractors.

youtube_dl/utils.py

@@ -1576,9 +1576,9 @@ US_RATINGS = {
 
 def parse_age_limit(s):
     if s is None:
-        return 0
+        return None
     m = re.match(r'^(?P<age>\d{1,2})\+?$', s)
-    return int(m.group('age')) if m else US_RATINGS.get(s, 0)
+    return int(m.group('age')) if m else US_RATINGS.get(s, None)
 
 
 def strip_jsonp(code):