Re-add support for modded biomes. Fixes #672.

Another change lost in the 1.5.1 update, now re-added. Fixes
ExtraBiomesXL crash on player connect, "Missing Biome mapping"
Add missed WorldGenerator vanilla compatibility. Fixes ExtraBiomesXL.

Also lost in 1.5.1 update. Required for ExtraBiomesXL compatibility. #672