[B] Corrected solid blocks in Material. Fixes BUKKIT-5671 & BUKKIT-5559

isSolid() returns nonsolid blocks (WALL_SIGN, IRON_PLATE, GOLD_PLATE, WOOD_PLATE and STONE_PLATE) as being solid and doesn't include FLOWER_POT as being solid.

JIRA Tickets:
[BUKKIT-5671] (https://bukkit.atlassian.net/browse/BUKKIT-5671)
[BUKKIT-5559] (https://bukkit.atlassian.net/browse/BUKKIT-5559)