Improve RestClient rate limiting

- Update the RestClient to handle retry-after headers in HTTP-date
  format (like 'Fri, 31 Dec 1999 23:59:59 GMT'). In this case, it will
  determine the delay by subtracting the date header from the
  retry-after header.
- Fix a bug in the RestClient.is_absolute_limit function, which was
  causing the RestClient to incorrectly categorize 413s due to a rate
  limit as absolute limits.

Change-Id: Ia7d0296a807e552f1766264ae4c98a3bbc641306
Closes-Bug: #1566000
2 files changed