Fix is_polynomial() so it works as advertised in the manual.

Fix is_polynomial() so it works as advertised in the manual.

GiNaC manual says:
"Testing whether an expression is a polynomial in one or more variables
can be done with the method

 bool ex::is_polynomial(const ex & vars) const;
In the case of more than one variable, the variables are given as a list.

(x*y*sin(y)).is_polynomial(x)        // Returns true.
(x*y*sin(y)).is_polynomial(lst(x,y)) // Returns false."

However, the implementation is a bit inconsistent, as GiNaC reports
sin(x) + 2*s     is a polynomial in s, but
pow(2,x) + 2*s   is not a polynomial in s.

This patch fixes the inconsistency. Thanks to Jonathan Cross for reporting
this bug.