Add test from KX for issue related to unbndfun.sum().
Add test from KX for issue related to unbndfun.sum().

The changes in the preceding commits close #496, and this commit adds a
check for that.  Also, disable an erroneously-issued warning about slow
decay.

asgeirbirkis/chebfun,alshedivat/chebfun,ilovejs/chebfun