scheduler/ipp.c: Convert incoming ColorModel attribute

[zdohnal]

If a client uses PPD option ColorModel (which is converted into attribute to be sent via IPP) for changing the default color option (which happens via CUPS-Add-Modify-Printer operation), we have to convert ColorModel into print-color-mode attribute, so the print-color-mode-default attribute gets updated.

The change survives reboot because print-color-mode is saved as an option in printer's entry in /etc/cups/printers.conf.