Include container name in X-Object-Manifest header when creating DLO

This is related to #258, and fixes the following error when PUTting Dynamic Large Objects (multipart files):

ClientException: Object PUT failed: https://storage101.dfw1.clouddrive.com:443/v1/MossoCloudFS_fab7bcd7-c313-4e3a-a55c-dea7ee252b1c/jordan-test/test 400 Bad Request   X-Object-Manifest must in the format container/prefix