Use "purge-job" instead of "purge-jobs" when canceling a single job (#742)

Use "purge-job" instead of "purge-jobs" when canceling a single job (#742)

The command "cancel -x <job>" adds "purge-jobs true" to the Cancel-Job
operation; however, the correct attribute to use for Cancel-job is
"purge-job" (singular), not "purge-jobs" (plural).  As a result, job
files are not removed from /var/spool/cups when "cancel -x <job>" is
executed.

This patch resolves the issue by adding "purge-job" when the IPP
operation is Cancel-Job and "purge-jobs" for other IPP operations
(Purge-Jobs, Cancel-Jobs, and Cancel-My-Jobs)