type_caster<std::function>: allow None values in both directions

example/example-callbacks.cpp

@@ -38,7 +38,10 @@ py::cpp_function test_callback5() {
 int dummy_function(int i) { return i + 1; }
 int dummy_function2(int i, int j) { return i + j; }
 std::function<int(int)> roundtrip(std::function<int(int)> f) { 
-    std::cout << "roundtrip.." << std::endl;
+    if (!f)
+        std::cout << "roundtrip (got None).." << std::endl;
+    else
+        std::cout << "roundtrip.." << std::endl;
     return f;
 }
 

example/example-callbacks.py

@@ -68,6 +68,8 @@ from example import roundtrip
 
 test_dummy_function(dummy_function)
 test_dummy_function(roundtrip(dummy_function))
+if roundtrip(None) is not None:
+    print("Problem!")
 test_dummy_function(lambda x: x + 2)
 
 try:

example/example-callbacks.ref

@@ -30,6 +30,7 @@ Copy constructions: 1
 Move constructions: True
 argument matches dummy_function
 eval(1) = 2
+roundtrip (got None)..
 roundtrip..
 argument matches dummy_function
 eval(1) = 2