Fix bug 344: always keep attributes of existing properties.

test/mjsunit/regress/regress-334.js

 // Copyright 2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 70 matching lines @@
 assertArrayEquals(["bar", "baz", "bif"], enumerable(object), "enum3");
 
 // Unshadowing a prototype property exposes its attributes.
 assertTrue(delete object.bif, "delete bif");
 assertArrayEquals(["bar", "baz"], enumerable(object), "enum4");
 assertEquals(func1, object.bif, "read bif 2");
 // Can't delete prototype property.
 assertTrue(delete object.bif, "delete bif 2");
 assertArrayEquals(["bar", "baz"], enumerable(object), "enum5");
 assertEquals(func1, object.bif, "read bif3");