Bug Fix: The S3 server throws 403 error if Date Header for a S3GetObjectRequest is more
than 15 minutes old.

When the S3TransferManager is used, the date property for a S3Request is set when the
downloadFile:bucket:key: is made rather than when the download begins.  For large multi file
downloads, this causes any download started 15 minutes after the initial download to fail
with a 403 error.

See: https://github.com/aws/aws-sdk-ios/issues/43
Merge pull request #44 from jasdeepsaini/master

Bug Fix: The S3 server throws 403 error if Date Header for a S3GetObject...