CategoryServiceTest: fixed variables' namees.

Should be in 54c9ab9a899c1afbc5f7021f2c8db55a34b6a27d commit.

No functional changes.
/series/add: fetch only id and name of categories instead of full object.

Minor SQL optimization.

No functional changes.
/series/add: fetch only id and name of countries instead of full object.

Minor SQL optimization.

No functional changes.