Added additional header (expires) returned by AWS::S3::Client#head_object and #get_object

Further to the headers added as a result of [issue 86](https://github.com/aws/aws-sdk-ruby/issues/86), I've added support for reading the Expires header and making it accessible in the same way as was done in commit https://github.com/aws/aws-sdk-ruby/commit/19b3864bfcd76892b6b1230621c4079a4ce372c9.