simplefeed_item_expires doesn't exist - lead to deletion by default of certain items

simplefeed_item.module tries to access simplefeed_item_expires when it doesn't find a valid expire value. However, this variable doesn't exist - I think it's just a typo and that we really want to access simplefeed_expires, which is set globally for every item in the administration form of simplefeed.

Attached is a patch which makes the (simple) changes. We only need to confirm that my hypothesis is right :)