/hreq did not check shulker box content

[lucsirotouka]

Will cause player sell empty shulker box to buyer.

To reproduce the problem:

1. Player A use /hreq to create a requisition, requested item is a shulker box with items (e.g. 1728 bones).
2. Player B use /hsell to sell an empty shulker box (with same color).
3. The sell operation will complete successfully, player B will get the money, but player A will only get an empty shulker box.