Save ETag when updating metadata (#5626)

Fixes #5622

cmd/fs-v1.go

@@ -403,6 +403,7 @@ func (fs *FSObjects) CopyObject(srcBucket, srcObject, dstBucket, dstObject strin
 		}
 
 		fsMeta.Meta = srcInfo.UserDefined
+		fsMeta.Meta["etag"] = srcInfo.ETag
 		if _, err = fsMeta.WriteTo(wlk); err != nil {
 			return oi, toObjectErr(err, srcBucket, srcObject)
 		}