Maxima library interface is broken for symbolic sums

[charpent]

sage: var("j,p", domain="integer")
(j, p)
sage: X=function("X")

Compare :

sage: foo=sum(X(j),j,1,p);foo
sum(X(j), j, 1, p)
sage: bar=maxima("sum(X(j),j,1,p)").sage();bar
sum(X(j), j, 1, p)
sage: bool(foo==bar)
True

with

sage: gee=X(j).maxima_methods().sum(j,1,p);gee
p*X(j)                   ## Nonsense...
sage: bool(foo==gee)
False

The problem also exists with products, see #17502 and #17505.

[rws]

The product part is #17502.