Fix issue 720 making Object.defineProperty handle existing writable flags correctly.

Fix issue 720 making Object.defineProperty handle existing writable flags correctly.

The existing version will overwrite the existing writable flag with
false even in the case where no new value is given and the existing
writable flag is true.

The reason for the issue is that there is no check to see if the
provided descriptor actually has a writable attribute. This causes us
to use the default value (false) even in the case where nothing was
provided. In addition, the existing tests makes wrong assumptions (that writable is always set to false if not provided) and has been changed to follow the specification.

Review URL: http://codereview.chromium.org/2271001

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4720 ce2b1a6d-e550-0410-aec6-3dcde31c8c00