If item exists, update on save, rather than delete and re-add

Deleting and readding the keychain was causing me some strange behavior, such as not being able to find the new values immediately.  It may have been an issue with security caching entitlements, regardless this fixed it for me.

There's one bit of code at line 51 where the kSecAttrSynchronizable is removed from the query,
 if you don't do this SecItemUpdate will work fine, and update the keychain attars correctly, but will still return a status of -34018.

This may clear up issues discussed in #52.