bug in hilbert_polynomial

[8d1h]

Hi! The function hilbert_polynomial sometimes gives wrong result.

julia> R, xi = grade(PolynomialRing(QQ, ["x$i" for i in 1:4])[1]);

julia> hilbert_polynomial(quo(R, ideal(R, sum(x^4 for x in xi)))[1]) # should be 2*t^2 + 2
2*t^2 + 6

I think the following line

Oscar.jl/src/Rings/mpoly-graded.jl

Line 637 in 1911457

for i=0:d

should be changed to for i=1:d+1 (also the array b declared several lines below seems unnecessary)