Fixes Hydroponics2 Icons

Created by: Kurfursten

Following @Deitylink 's example, I started adding new plants to hydroponics2.dmi - but in testing I noticed these plants disappeared when modified (e.g.: with Robust Harvest). This was because diverge() did not copy over plant_dmi, as I discovered with the aid of @9600bauds

This fixes it so that those plants will not vanish leaving seemingly empty trays. Fully tested and includes changelog.