Fix create archive to a continuous writing source file failed

#387

archiver.go

@@ -221,7 +221,7 @@ func openAndCopyFile(file File, w io.Writer) error {
  return err
  }
  defer fileReader.Close()
- _, err = io.Copy(w, fileReader)
+ _, err = io.CopyN(w, fileReader, file.Size())
  return err
 }