[PATCH 1/2] extract_integer_content: check for rational numbers properly.

[PATCH 1/2] extract_integer_content: check for rational numbers properly.

Commit 3d09388a (titled as `[bugfix] chinrem_gcd: handle polynomials over
rationals properly.') broke extract_integer_content: now it always returns 1.
The check for rational `integer_contnent' introduced by that commit is wrong
(since integers is a subset of rationals). Rewrite the check proprerly.
(cherry picked from commit f0fb303711b4334ce59f7da63bfa7cb49f46265f)