Series.diff returned with wrong x0

The following example displays how Series.diff returns an object of the correct type, with the correct value, but with the wrong x0:

>>> import numpy
>>> from gwpy.timeseries import TimeSeries
>>> a = TimeSeries(numpy.arange(10))
>>> a.diff()
<TimeSeries([1,1,1,1,1,1,1,1,1]
            unit=None,
            name=None,
            channel=None,
            x0=<Quantity 0.0 s>,
            dx=<Quantity 1.0 s>)>

The x0 value should really by 1s.