Insert missing type cast in JSON.stringify.

R=dcarney@chromium.org
BUG=v8:2570

Review URL: https://chromiumcodereview.appspot.com/12599003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13853 ce2b1a6d-e550-0410-aec6-3dcde31c8c00