fix an incorrect OpenMode flags handling in QBuffer::open()

fix an incorrect OpenMode flags handling in QBuffer::open()

which leads to absurd statement (QBuffer::open() == !QBuffer::isOpen()) to be true.
also treat Truncate as (Truncate | WriteOnly) to satisfy lazy ones

Reviewed-by: Joao
Merge-request: 2612
(cherry picked from commit 6b91affb9a355e668bc9d06dee580d95230ac63a)

Change-Id: I657d4d0a33f7993313fe2a1a8ba408371991717f
Reviewed-on: http://codereview.qt.nokia.com/447
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Peter Hartmann <peter.hartmann@nokia.com>