Fix: Preserve MD5Sum for SSE encrypted objects (#6680)

To conform with AWS S3 Spec on ETag for SSE-S3 encrypted objects,
encrypt client sent MD5Sum and store it on backend as ETag.Extend
this behavior to SSE-C encrypted objects.

cmd/object-api-datatypes.go

@@ -108,6 +108,8 @@ type ObjectInfo struct {
 	// Implements writer and reader used by CopyObject API
 	Writer       io.WriteCloser `json:"-"`
 	Reader       *hash.Reader   `json:"-"`
+	PutObjReader *PutObjReader  `json:"-"`
+
 	metadataOnly bool
 
 	// Date and time when the object was last accessed.