Case C-185/15: Judgment of the Court (Third Chamber) of 12 October 2016 (request for a preliminary ruling from the Vrhovno sodišče — Slovenia) — Marjan Kostanjevec v F&S Leasing GmbH (Reference for a preliminary ruling — Judicial cooperation in civil matters — Regulation (EC) No 44/2001 — Article 6(3) — Definition of ‘counterclaim’ — Claim based on unjust enrichment — Payment of a sum due under a decision that has been set aside — Temporal application)