TST: fix _infer_freq for pandas .14+ compat

closes https://github.com/statsmodels/statsmodels/issues/1822
some discussion over at pandas: https://github.com/pydata/pandas/issues/7922

Basically pandas 0.14 fixed a bug with DatetimeIndex's raising an AttributeError if you
tried to access its freqstr attribute when it doesn't have a frequency.
The tests previously relied on having the freqstr attribute implying that freqstr was not None.