reapply the tests for nan cases

Author: peterpanmj

pandas/tests/series/test_rank.py

@@ -196,9 +196,6 @@ def test_rank_signature(self):
         pytest.raises(ValueError, s.rank, 'average')
 
     def test_rank_inf(self):
-        pytest.skip('DataFrame.rank does not currently rank '
-                    'np.inf and -np.inf properly')
-
         values = np.array(
             [-np.inf, -50, -1, -1e-20, -1e-25, -1e-50, 0, 1e-40, 1e-20, 1e-10,
              2, 40, np.inf], dtype='float64')