Special methods (__foo__) don't work as expected on ProxyObject methods

Considering Python's duck-typing semantics, it'd be really useful if it were possible to define custom 'special functions' (like __add__, __str__, etc) on ProxyObject, in order to build custom objects that mesh nicely with the rest of Python.

Defining these methods is currently possible, but Python doesn't pick them up properly. For instance, consider the following code (pardon the Clojure ;)):

(import '[org.graalvm.polyglot Context] '[org.graalvm.polyglot.proxy ProxyObject ProxyExecutable])
(let [ctx (Context/create (into-array String ["python"]))
      obj (ProxyObject/fromMap {"__len__" (reify ProxyExecutable (execute [_ _] 1))
                                "__add__" (reify ProxyExecutable (execute [_ _] 2))
                                "__str__" (reify ProxyExecutable (execute [_ _] "hi"))})]
  (.putMember (.getBindings ctx "python") "x" obj)
  (.eval ctx "python" "print(x.__len__(), x.__add__(1), x.__str__())")
  (.eval ctx "python" "print(x)")
  (try (.eval ctx "python" "print(len(x))") (catch Exception e (println (ex-message e))))
  (try (.eval ctx "python" "print(x + 1)") (catch Exception e (println (ex-message e)))))

The expected output here should be

1 2 hi
hi
1
2

but in reality, what we get is:

1 2 hi
<foreign object at 0x67b613f9>
AttributeError: foreign object has no attribute '__len__'
TypeError: unsupported operand type(s) for +: 'foreign' and 'int'

Start by reproducing the example using Context/create, ProxyObject/fromMap, and the Python eval calls. Trace how ProxyObject members are exposed to Python's special-method lookup, then verify that str, len, and add work through print(), len(), and + with the expected output.