When there are interceptors on an object, it's possible to
end up with duplicate property names. But when all the names
are provided by v8, a collision is not possible, so we can
fast-path that case by not de-duping.

Also added better test coverage for interceptor API.

Review URL: https://codereview.chromium.org/12314081

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13725 ce2b1a6d-e550-0410-aec6-3dcde31c8c00