python3 test_value_counts_nunique  : TypeError: unhashable type: 'NaTType'
on sparc   4c2d050

```
======================================================================
ERROR: test_value_counts_nunique (pandas.tests.test_series.TestSeries)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/yoh/deb/gits/pkg-exppsy/pandas/debian/tmp/usr/lib/python3/dist-packages/pandas/tests/test_series.py", line 2633, in test_value_counts_nunique
    result = s.value_counts()
  File "/home/yoh/deb/gits/pkg-exppsy/pandas/debian/tmp/usr/lib/python3/dist-packages/pandas/core/series.py", line 1422, in value_counts
    normalize=normalize)
  File "/home/yoh/deb/gits/pkg-exppsy/pandas/debian/tmp/usr/lib/python3/dist-packages/pandas/core/algorithms.py", line 186, in value_counts
    keys, counts = htable.value_count_object(values, mask)
  File "hashtable.pyx", line 935, in pandas.hashtable.value_count_object (pandas/hashtable.c:14937)
TypeError: unhashable type: 'NaTType'

```
