Added __contains__ to stl bindings for maps (#1767)

* Added __contains__ to stl bindings for maps
diff --git a/tests/test_stl.py b/tests/test_stl.py
index bf185d5..2335cb9 100644
--- a/tests/test_stl.py
+++ b/tests/test_stl.py
@@ -56,7 +56,9 @@
     """std::map <-> dict"""
     d = m.cast_map()
     assert d == {"key": "value"}
+    assert "key" in d
     d["key2"] = "value2"
+    assert "key2" in d
     assert m.load_map(d)
 
     assert doc(m.cast_map) == "cast_map() -> Dict[str, str]"