Format timestamp headers correctly

HTTP header fields with timestamps do not use the formatting specified in the api definition, but use formatting with the local timezone.
This breaks the ifModifiedSince parameter for AWS.S3.getObject for me, because it sets If-Modified-Since to "Jul 09 2013 14:01:46 GMT+0200 (CEST)" (which the server apparently cannot parse) instead of using the required rfc822 formatting ("Tue, 09 Jul 2013 12:04:26 GMT")