QNetworkHeadersPrivate::toHttpDate() used a custom format to output a
date-time; the format supplied GMT as suffix, but neglected to convert
the date-time to UTC, so local-time was formatted as if it were UTC,
regardless of its actual offset from it. Fixing this (by the obvious
toUTC() call) broke formatting when the supplied header value was a
QDate, since it's packaged as a QVariant and QVariant's conversion of
QDate to QDateTime uses local time's (not UTC's) start of day. So fix
headerValue() to separate QDate and QDateTime cases and use
startOfDay(Qt::UTC) to get the right start of the day. Added tests for
non-UTC date-times appearing correctly in HTTP headers.
Fixes: QTBUG-80666
Pick-to: 6.3 6.2 6.2.3 5.15
Change-Id: I2792bce14a07be025cf551b0594630260c112269
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
aa8fd6bc39