Preserve modification and access times

[freayd]

Hi there,

Could we add an option to download files while preserving their modification and access times? A flag recursive: true added to the method Net::SFTP::Session#download would be nice to have. It could work like the -p option of the sftp command line.

Here's my quick and dirty fix:

files = []
sftp.download!("destination", "local", recursive: true) do |event, downloader, *args|
  case event
  when :close then
    files << args[0]
  when :finish then
    files.each { |file| File.utime(sftp.stat!(file.remote).atime, sftp.stat!(file.remote).mtime, file.local) }
  end
end

I can work on a pull request if you support the idea.