Fix warning: content-type missing in HTTP POST, defaulting to application/x-www-form-urlencoded. Use QNetworkRequest::setHeader() to fix this problem.

Summary:
When downloading/updaring data from the internet (comet, asteroids etc..) the http post command misses the setHeader "application/x-www-form-urlencoded" string.
This simple patch fixes the warning:
content-type missing in HTTP POST, defaulting to application/x-www-form-urlencoded. Use QNetworkRequest::setHeader() to fix this problem.

Reviewers: mutlaqja, lancaster

Reviewed By: mutlaqja

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D15312