Make a better test for the array_is_pmf question

[charlesfrye]

Right now, doesn't appropriately test whether there are values above 1 or below 0 because the examples don't add up to 1, which is what most folks test for.

np.array([-1, 2]) and/or np.array([-0.5, 1.1, 0.2, 0.2]) would do it.