writable books do not sign when you hit the sign button.

additionally when a writable book or written book is dropped and then picked up, or moved to a different inventory slot, the nbt disappears, causing all pages and the title of the book to be lost.

This bug does not occur in vanilla and does not seem to occur in version

git-Spigot-6505f06-322f2f9 (MC: 1.13-pre7)

of spigot.

This behavior occurs with no running plugins.

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