[Rackspace|Storage] last_modified= work around

This pull request addresses issue #1809 where `get_container` request method returns a last_modified timestamp similar to the following `2013-05-09T22:20:59.287990`.

Ruby assumes this date is in the local time zone (which is the appropriate behavior according to iso 8601), however, cloud files (swift) is returning the date in UTC.

https://github.com/openstack/swift/blob/master/swift/container/server.py#L398-L399

An initial fix was proposed in PR #1811 by @neojin. The hope is that this PR is a little more robust.