class Zip::File

def mkdir(entry_name, permission = 0o755)
  raise Errno::EEXIST, "File exists - #{entry_name}" if find_entry(entry_name)
  entry_name = entry_name.dup.to_s
  entry_name << '/' unless entry_name.end_with?('/')
  @entry_set << ::Zip::StreamableDirectory.new(@name, entry_name, nil, permission)
end