Arvados

keepstore does not include a `Content-Length: 0` header when attempting to create empty objects using the S3 API.

This is because an io.Reader is being created and passed (via goamz) to net.http even when the data is nil.

According to the net.http.Request.ContentLength documentation:

        // For client requests, a value of 0 with a non-nil Body is
        // also treated as unknown.

This means that if a non-nil io.Reader is given, a `ContentLength` of 0 will result in omitting the `Content-Length` header from the request, which results in requests to put empty objects to S3-like services such as Ceph RadosGW to fail (with HTTP status 411).

A PR that addresses this issue will be forthcoming.