When I create a CUPS queue with a PPD file with choice names "Tray-1", "Tray-2",
... in the InputSlot option CUPS translates these names to double-dashed IPP
attribute names: "tray--1", "tray--2", ... in the "media-source" attribute, both
when passing a job to the printer with the IPP backend, making the printer
ignore the tray choice, and also when answering a get-printer-attributes IPP
request from a client. This happens when in the PPD a dash is followed by a
digit, as the pwg_unppdize_name() function in cups/ppd-cache.c inserts a dash
whenever a non-digit is followed by a digit in the PPD name. As IPP attribute
names generally never have double-dashes and also no dashes in the beginning or
the end of the name, I have modified the pwg_unppdize_name() function
appropriately.