Fix small spec violation in String.prototype.split.

Also slightly extended the test coverage.

R=rossberg@chromium.org
BUG=v8:3026
LOG=Y

Review URL: https://codereview.chromium.org/119093002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18405 ce2b1a6d-e550-0410-aec6-3dcde31c8c00