Fix issue with numeric property names

We were not correctly treating 1.0 as 1, nor 1.20 as 1.2 in accessors.

BUG=v8:3507
LOG=Y
R=marja@chromium.org

Review URL: https://codereview.chromium.org/493173003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23321 ce2b1a6d-e550-0410-aec6-3dcde31c8c00