Bug 17530: (QA follow-up) Fix caching error

Bug 17530: (QA follow-up) Fix caching error

The fourth patch made this change:
-    $last_article_requestable_guesses->{$key} = $res;
+    $cache->set_in_cache(GUESSED_ITEMTYPES_KEY, $res);

This removed the category level in the cached hash. Adding it back here.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Tested by verifying cached values, running opac-search with anonymous
session as well as user context.

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>