Bug: ex(numeric(0)).series(x==0,2).is_zero() returns incorrectly 0

[Pearu Peterson]

Hi!

The explanation for this bug is that pseries defines its own
is_zero() method that is not used within ex::is_zero() method.
As a possible solution:
  define basic::is_zero() (with the contents of current ex::is_zero()) and
redefine ex::is_zero() to call ex::bp->is_zero().

I guess that matrix class could also have its own is_zero() method.

Pearu