#190 Fixed resource leak when obtaining S3Object size

It appears that when a client requests for an object's partial contents, another `S3Object` is obtained in order to get its full length so that it can be used in the `Content-Range` header data. However, this `S3Object` is not closed, which causes a connection leak, as each `S3Object` reference creates a connection to Amazon S3. See https://stackoverflow.com/questions/17782937/connectionpooltimeoutexception-when-iterating-objects-in-s3

This should address the error reported in #190.