TST: Add test for gh_39

This tests whether IRR returns the expected positive rate rather
than a smaller but negative rate.

Author: Kai-Striega

numpy_financial/tests/test_financial.py

@@ -624,3 +624,16 @@ def test_gh_15(self):
         # Very rough approximation taken from the issue.
         desired = -0.9999999990596069
         assert_allclose(result, desired, rtol=1e-9)
+
+    def test_gh_39(self):
+        cashflows = numpy.array([
+            -217500.0, -217500.0, 108466.80462450592, 101129.96439328062,
+            93793.12416205535, 86456.28393083003, 79119.44369960476,
+            71782.60346837944, 64445.76323715414, 57108.92300592884,
+            49772.08277470355, 42435.24254347826, 35098.40231225296,
+            27761.56208102766, 20424.721849802358, 13087.88161857707,
+            5751.041387351768, -1585.7988438735192, -8922.639075098821,
+            -16259.479306324123, -23596.31953754941, -30933.159768774713,
+            -38270.0, -45606.8402312253, -52943.680462450604,
+            -60280.520693675906, -67617.36092490121])
+        assert_almost_equal(npf.irr(cashflows), 0.12)