Advanced Sign (still) does not support credit card buys correctly

[nrllewellyn]

Issue #26 mostly fixed the problem, but there's still a minor bug:

• Place an advanced sign
• Set up a vending block to buy items
• Add a credit card
• Close the GUI
• Open the GUI
• Close the GUI
• The sign now reads "Out Of Coins!"

(In other words, opening the GUI and not changing anything causes it to think it's out of coins)

[notabadminer]

This issue should be resolved with 1.7.x-1.6.38 release.