Revert "Revert "Minor fixes in sum@singfun.""
Revert "Revert "Minor fixes in sum@singfun.""

This reverts commit 075e982fb4f5d36d0c19cac1b379fd6f96a69926.

asgeirbirkis/chebfun,alshedivat/chebfun,ilovejs/chebfun