object: fix signed payload calculation of HeadRequest message

In previous implementation first byte of buffer for HeadRequest
signed payload was set to 1 if FullHeaders flag was set. Otherwise,
this byte remained unchanged. For correct recording of a signed payload,
it is necessary to explicitly set the first byte with the unset flag.

object/sign.go

@@ -82,6 +82,8 @@ func (m HeadRequest) ReadSignedData(p []byte) (int, error) {
 
 	if m.GetFullHeaders() {
 		p[0] = 1
+	} else {
+		p[0] = 0
 	}
 
 	off := 1