Test for new cut.call behavior

With the new behavior, cuts should return the events directly when called. This
change breaks existing code.

nnfwtbn/tests/test_cut.py

@@ -241,3 +241,15 @@ class CutTestCase(unittest.TestCase):
                          [True, True, False, False, False, True, True, False])
         self.assertEqual(list(self.df[selected].year),
                          [2010, 2011, 2015, 2016])
+
+    def test_call(self):
+        """
+        Check that calling the cut returns a dataframe containing the events
+        rather than returning an index array.
+        """
+        high_sale = Cut(lambda df: df.sale > 4)
+        high_sale_years = high_sale(self.df)
+
+        self.assertEqual(list(high_sale_years.sale), [4.7, 5.6, 7.5, 4.2])
+        self.assertEqual(list(high_sale_years.year), [2012, 2013, 2014, 2017])
+