With load_package specfn
zeta(-1) returns 1/12 instead of -1/12
zeta(-3) returns -1/180 instead of 1/120
etc.; it does not depend on numval and rounded switches.
Thanks for reporting this. I believe I found the reason for this erroneous behaviour, but I need to test this further. I'll commit a fix ASAP.
Fix committed (r1748)