secret-store: Set the schema name correctly on loaded items

 * When we loaded items from the keyring we didn't set the schema
   correctly.
 * This causes any searches for the item that include a schema
   in the search parameters to fail.
 * Also caused problems storing items, when it was expected that
   the item would replace any already stored. This uses a search
   internally.
 * Fix and add a test for this case, both for encrypted and
   plaintext keyring files.

https://bugzilla.gnome.org/show_bug.cgi?id=681727