swift updateMetadata resets the object's content-type

In the case of a null content-type, jclouds will set the content-type to application/x-www-form-urlencoded, probably in https://github.com/jclouds/jclouds/blob/master/core/src/main/java/org/jclouds/io/payloads/UrlEncodedFormPayload.java#L33