When determining ids of mails to delete a special identifier -1 is used to reference all e-mails. When the "leave on server" flag is not set asking for deletion of all mails should really return a list of all mail ids.
This fixes Pop3Test::testSimpleLeaveOnServer test case.
This patch is an effort to fix some POP3 autotests. I'm not deeply familiar with the POP3 resource, but the change seems logical.