tm.TestCase.assert_numpy_array_equal(...) and np.nan
Is this intended:

``` python
# In a testcase:
exp = np.array([1,2,4,np.nan])
self.assert_numpy_array_equal(exp, exp)

Traceback (most recent call last):
  File "C:\data\external\pandas\pandas\tests\test_categorical.py", line 490, in test_groupby_categorical_no_compress
    self.assert_numpy_array_equal(exp, exp)
  File "C:\data\external\pandas\pandas\util\testing.py", line 91, in assert_numpy_array_equal
    raise AssertionError('{0} is not equal to {1}.'.format(np_array, assert_equal))
AssertionError: [  1.   2.   4.  nan] is not equal to [  1.   2.   4.  nan].
```

?
