Fix the evaluation of polylog(s, -1)

It should be `polylog(1, z) = -log(1-z)` which gives for `z = -1`
the value `-log(2)`. In general:

  polylog(s, 1) = zeta(s)

and

  polylog(s, -1) = - dirichlet_eta(s)

This minus sign was missing!

References:

 http://en.wikipedia.org/wiki/Polylog#Relationship_to_other_functions
Fix expansion of polygamma(n, p/q) for n in Z and 1 <= p < q