fix the reference count for dicts test_get_default_not_found

Something is double-free-ed during the final finalization:  in a debug build of Python, `./Include/object.h:602: _Py_NegativeRefcount: Assertion failed: object has negative ref count`

In non-debug build of Python, it simply segfaults at the end during finalization.

src/JSObjectProxy.cc

@@ -637,6 +637,7 @@ PyObject *JSObjectProxyMethodDefinitions::JSObjectProxy_get_method(JSObjectProxy
 
   PyObject *value = JSObjectProxy_get(self, key);
   if (value == Py_None) {
+    Py_INCREF(default_value);
     value = default_value;
   }