Fix Object.assign to call [[GetOwnProperty]] Proxy handler only once (#3988)

Removed the enumerable option when listing properties to prevent an extra
[[GetOwnProperty]] function call for Proxy objects.
In this case all the property names are collected, but the Object.assign()
implementation has already taken care of the enumerable property filtering.

JerryScript-DCO-1.0-Signed-off-by: Roland Takacs rtakacs@inf.u-szeged.hu

tests/jerry/es.next/proxy_get_own_property_descriptor.js

@@ -268,3 +268,17 @@ try {
 }
 
 // Step 17: See (Inv-2) above.
+
+var result = [];
+var proxy = new Proxy({foo: 1, bar: 2}, {
+  getOwnPropertyDescriptor: function(o, v) {
+    result.push(v);
+    return Object.getOwnPropertyDescriptor(o, v);
+  }
+});
+
+Object.assign({}, proxy);
+
+assert(result.length === 2);
+assert(result[0] === "foo");
+assert(result[1] === "bar");