Consistently use a string type for expectedLayerDiffIDFlag

Might be a fix for containers#2602 .

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

storage/storage_dest.go

@@ -930,7 +930,7 @@ func (s *storageImageDestination) createNewLayer(index int, layerDigest digest.D
 
 		flags := make(map[string]interface{})
 		if untrustedUncompressedDigest != "" {
-			flags[expectedLayerDiffIDFlag] = untrustedUncompressedDigest
+			flags[expectedLayerDiffIDFlag] = untrustedUncompressedDigest.String()
 			logrus.Debugf("Setting uncompressed digest to %q for layer %q", untrustedUncompressedDigest, newLayerID)
 		}