Related to #1511

When updating an objectstorage account or object, it is not possible to remove a metadata. If you set a key to an empty string the empty header is stripped from the request when the POST is made.

// MoreHeaders specifies additional HTTP headers to be provide on the request. If a header is
// provided with a blank value (""), that header will be omitted instead: use this to suppress
// the default Accept header or an inferred Content-Type, for example.
MoreHeaders map[string]string

Both calls should have a RemoveMetadata slice to allow sending X-Remove-xxx-Meta-xxx headers.