Fix Proxy ownKeys() postprocess array index bug

Triggered when Array.prototype has index properties, e.g. a setter.

src-input/duk_bi_proxy.c

@@ -63,7 +63,9 @@ DUK_INTERNAL void duk_proxy_ownkeys_postprocess(duk_hthread *thr, duk_hobject *h
 		}
 
 		/* [ obj trap_result res_arr propname ] */
-		duk_put_prop_index(thr, -2, idx++);
+		duk_push_uarridx(thr, idx++);
+		duk_insert(thr, -2);
+		duk_def_prop(thr, -3, DUK_DEFPROP_HAVE_VALUE | DUK_DEFPROP_SET_WEC);
 		continue;
 
 	 skip_key:

tests/ecmascript/test-bug-proxy-ownkeys-arridx-inherit-gh2207.js

@@ -0,0 +1,17 @@
+// https://github.com/svaarala/duktape/issues/2207
+
+/*===
+{"foo":0,"nonEnumerable":0}
+done
+===*/
+
+Object.defineProperty(Array.prototype, 0, { set: function () { } });
+
+function jsonStringifyOwnKeysProxyTest () {
+  target = { foo: 0, nonEnumerable: 0 };
+  proxy = new Proxy(target, { $: function () { }, ownKeys: function () { return [ 'foo', 'nonEnumerable' ] } });
+  print(String(JSON.stringify(proxy)));
+}
+
+jsonStringifyOwnKeysProxyTest();
+print('done');