LibJS: Object::put() shouldn't create own properties on prototype chain

Unless there is a setter present on the prototype chain, put() should
only set own properties on the object itself.

Fixes #1588.

Libraries/LibJS/Tests/Object.prototype.toString.js

@@ -0,0 +1,8 @@
+function assert(x) { if (!x) throw 1; }
+
+try {
+    assert(typeof Object.prototype.toString() === "string");
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}