Fix bug in chebfun.plus().
Fix bug in chebfun.plus().

Cherry-pick 7ca813ce into this branch.  This fix is needed here because
chebfun.sum() relies on chebfun.minus() (and hence chebfun.plus()) to do
some of its work.

ilovejs/chebfun,asgeirbirkis/chebfun,alshedivat/chebfun