aggregator_block_save fails with "Field 'description' doesn't have a default value"

When trying to update the number of feed items in the block configuration of a aggregator category, it fails with the following Exception:

PDOException: INSERT INTO {aggregator_category} (block, cid) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1) ON DUPLICATE KEY UPDATE block=VALUES(block) - Array ( [:db_insert_placeholder_0] => 4 [:db_insert_placeholder_1] => 2 ) SQLSTATE[HY000]: General error: 1364 Field 'description' doesn't have a default value in aggregator_block_save() (line 339 of .../aggregator.module).

I think the easiest way is to replace db_merge() with db_update(), simply because we don't need db_merge() in that scenario.

- If we configure a block of a aggregator category, we can assume that the aggregator category exists.
- If the category is for some reason *not* available (it might have been deleted), db_update() will silently do nothing, which makes sense. db_merge() would either fail because not all needed fields are available (like how it's doing now) or re-create the category. I don't think that should happen.