FrequencySeries().interpolate().pad().frequencies has wrong length

Created by: lpsinger

Interpolating and then padding a FrequencySeries causes its .frequencies array to have the wrong length.

>>> from gwpy.frequencyseries import FrequencySeries
>>> a = FrequencySeries(range(10)).interpolate(1).pad(5)
>>> len(a), len(a.frequencies)
(20, 10)