multi_delete: make content-md5 mandatory

The AWS S3 document says that the content-md5 header must be used for the
Multi-Object Delete operation.  This patch add a change to follow the
specification.

Change-Id: I0b289a0302e6465a23acb0acbfa1636b1ef91e5b
Merge "multi_delete: make content-md5 mandatory"