Fix negative damage from Zombies. Fixes BUKKIT-4193

Currently, the method used for calculating the damage of zombies is scaled
to their health, but it uses the default max health rather than the real
max health value. If zombies have more health than the default max health
value, the amount of damage they deal becomes negative.

This is caused by EntityZombie.getMaxHealth() returning a hardcoded value of
20, which is the vanilla max health for zombies. Rather than using this value
when calculating zombie damage, the call is changed to instead use
((CraftLivingEntity) this.bukkitEntity).getMaxHealth(). This uses the true
maximum health of the Entity. "this.maxHealth" could be used instead of the
aforementioned method, however that creates a very unclear diff, and a
confusing change.
Correct cancellation of InventoryClickEvent. Fixes BUKKIT-4331

Cancelling an InventoryClickEvent for a single click causes the click not
to be processed by the clicked inventory. The server then doesn't
correctly identify their second click as being a double click, causing an
inconsistency between what the Player believes the inventory to be and
what the server believes the inventory to be.

This change forces an updateInventory call whenever an InventoryClickEvent
whose action is NOTHING is cancelled.  Both clicks are considered
PICKUP_ALL, so updating the inventory after the second click fixes any
inconsistencies that could arise between the client and the server.
Fix cancellation for InventoryDragEvent. Fixes BUKKIT-4332

Cancelling InventoryDragEvent causes the placed items to be lost.  This is
because the cursor is set to the result prior to the event taking place,
so the items are removed from the cursor. When the event is cancelled, the
items removed from the cursor aren't placed in the inventory, and are just
lost.

This change sets the cursor back to the original cursor when the event is
cancelled.
Updated version to 1.5.2-R1.0 in pom.xml for RB.
Updated version to 1.5.2-R1.1-SNAPSHOT for development towards next release.