Although not defined, the jpeg comment field 'content' is treated as
utf-8 nowadys. At least exiftool and exiv2 (tested via gwenview) are
expecting utf-8 here. So we should do the same.
Task-number: QTBUG-44709
Change-Id: If84dafac3e337c7993f09cd59792e721977c9adb
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>
