Fix S3 Bucket Endpoint URL for us-east

For me, running any request to a bucket on us-east results in

    Get : 301 response missing Location header

Because AWS returns incorrect 301 responses, which go does not expose.
When hitting the bucket with `curl https://s3.amazonaws.com/<bucket-name>`, S3 responds with the correct endpoint. This patch fixes the endpoint, at least for us-east.