Fix PlayerFishEvent not correctly cancelling. Fixes BUKKIT-5396

Prior to this commit cancelling the PlayerFishEvent would cause various
states of the fishing routine to be incorrectly or wrongly fired. This
incorrect behaviour was due to the miscommunication between the server and
client regarding the fishing state. When the event was cancelled, the
bobber entity was removed and caused the client to incorrectly determine
what the "next state" was to logically be.

This commit resolves the issue by ensuring the client is made aware of the
correct changes at the correct time regarding the bobber entity, therefore
keeping the logical steps of "fishing" proper and in-tact.
Modify the invalid item set to permit command blocks. Fixes BUKKIT-4342

When using a "vanilla" Minecraft server using the "pick block" key on a
command block yields the invoker with a command block within their
inventory while in creative mode. Implications of the invalid items set
containing the command block also include having a "ghost" item that
cannot be placed due to it not actually existing.

This commit resolves the problem and brings Craftbukkit closer to vanilla
behaviour by removing the command block item ID, 137, from the invalid
items set.