Fixes #1

  slightly ignore incorrect response when querying magnet tasks,
  such that we dont panic on this issue.

  cannot figure out why lixian returns incorrect response, in which `size` is reported as `NONE`, when add a magnet task:

    magnet:?xt=urn:btih:CCFA2C5E9E33DC93975C35A0F755DAE814B39C00&dn=penthouse+usa+october+2013&tr=udp%3A%2F%2Ftracker.publicbt.com%3A80%2Fannounce&tr=udp%3A%2F%2Fopen.demonii.com%3A1337

  use uploading .torrent for such tasks.