Make QStringRef::right() consistent with QString::right()

The implementation was inconsistent with QString::right(),
and did not return the N rightmost characters but actually did
the same as QString::mid(N) (returning the rightmost size - N
characters.)

Since this function is fairly recent (Qt 5.2), is documented to
behave the same as QString::right(), and since these APIs are
meant to be interchangeable, this needs to be fixed, even though
it changes behavior.

[ChangeLog][Important Behavior Changes] Changed QStringRef::right()
to be consistent with QString::right(). The function now returns
the N right-most characters, like the documentation already claimed.

Change-Id: I2d1cd6d958dfa9354aa09f16bd27b1ed209c2d11
Task-number: QTBUG-41858
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>