BUG/TST: Precision on downcast

[TomAugspurger]

See #5174

May not be just Macs. From that thread, yarikoptic's OS was: OS: Linux 3.9-1-amd64 #1 SMP Debian 3.9.8-1 x86_64.

A sample failure is from the generic.interpolate suite (modified to downcast, which is the default):

    def test_interp_quad(self):
        sq = Series([1, 4, np.nan, 16], index=[1, 2, 3, 4])
        result = sq.interpolate(method='quadratic')
        expected = Series([1., 4, 9, 16], index=[1, 2, 3, 4])
        assert_series_equal(result, expected)

On (some?) macs and apparently some linux builds this will fail because it the result is not downcast to an int. See also stepping through here and the PR to patch this failure here.

I'm going to see if I can get my failing scipy/numpy tests to pass cleanly. Hopefully that will fix these.

[jreback]

closed by #5368