Fix a problem where Object.getOwnPropertyDescriptor and related functions unintentionally called toString on the values of an object's properties.  Fixes issue 1233. 
Review URL: http://codereview.chromium.org/6677017

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7151 ce2b1a6d-e550-0410-aec6-3dcde31c8c00