TST: fix _infer_freq for pandas .14+ compat

pandas 0.14 fixed a bug with DatetimeIndex's
freqstr attribute. Previously relied on
having the freqstr attribute implying that freqstr
was not none.
Merge pull request #1872 from TomAugspurger/infer_freq_pandas_break

BUG: fix _infer_freq for pandas .14+ compat closes #1822