KIMAP fake server: add support for disconnect before response

Summary: well, the support is there, it's just that the assert prevented it.

Test Plan: This is going to be used in kdepim-runtime shortly.

Reviewers: ervin

Reviewed By: ervin

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D12400