always close the downloaded file after downloading

Summary:
sometimes the downloaded file isn't correctly flushed to
disk, causing corrupted zip files, especially if they come
from github that does a redirect. Unconditionally closing it
fixes the problem

Test Plan:
downloaded the unity8 gtk3 theme that comes from github and is fine,
other files coming directly from store.kde.org are fine as well

Reviewers: Plasma, leinir, bshah

Reviewed By: leinir

Subscribers: plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D5402