[bugfix] ex::is_polynomial(x): fix miscomputations

[bugfix] ex::is_polynomial(x): fix miscomputations

The function power::is_polynomial(x) sometimes returned false,
even if x did not occur in the power object.

Thanks to Rubel Zilibowitz for reporting this bug.

Similarly, mul::is_polynomial(x) sometimes returned true, even
if x occurred only in the denominator of a rational function.

The function mul::is_polynomial(x) does a simpler version of
power::is_polynomial(x) on its factors. (It's simpler because the
exponents cannot be functions of x.)