* inline: When previewing, still call will_render on rss/atom files,

* inline: When previewing, still call will_render on rss/atom files,
  just avoid actually writing the files. This is necessary because ikiwiki
  saves state after a preview (in case it actually *did* write files),
  and if will_render isn't called its security checks will get upset
  when the page is saved. Thanks to Edward Betts for his help tracking this
  tricky bug down.