Don't fetch missing parts from the resource.
Summary:
Dan wrote this line of code, not me ;)
"Other resources are requesting ENVELOPE, but not every resource
provides ENVELOPE" (only emails have that)
Test Plan: Saving changes in FatCRM works again
Reviewers: dvratil
Reviewed By: dvratil
Subscribers: winterz, kde-pim
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D14853