Various TTL fixes

1. Utility.parse_time was ignoring time zones. This was causing robots.txt to sometimes get fetched excessively.
2. The no-cache Cache-Control: directive should only be honored if it appears "bare", not in no-cache=blah form (in which case it has another meaning).
3. If the expires header cannot be parsed, use the default TTL instead of a TTL of 0.