Add regression tests

Closes #509

mpmath/tests/test_fp.py

@@ -1680,3 +1680,4 @@ def test_fp_nan_in_args():
     assert fp.isnan(fp.e1(fp.nan))  # issue 487
     assert fp.isnan(fp.chebyt(1.3, fp.nan))  # issue 478
     assert fp.isnan(fp.chebyt(13, fp.nan))
+    assert fp.isnan(fp.hyp2f2(0.4, 2.5, 2.2, 0.7, fp.nan))  # issue 509

mpmath/tests/test_functions.py

@@ -919,3 +919,4 @@ def test_mp_nan_in_args():
     assert mp.isnan(mp.hyp1f1(2.5,2.2, mp.nan))  # issue 488
     assert mp.isnan(mp.hyp1f1(1,2.2, mp.nan))
     assert mp.isnan(mp.hyp1f1(1,2002.2, mp.nan))
+    assert mp.isnan(mp.hyp2f2(0.4, 2.5, 2.2, 0.7, mp.nan))  # issue 509