Fix pow(+(...),2).expand().

Fix pow(+(...),2).expand().

Due to a failure to recombine coeffs and rests to expairs,
expand((x+sqrt(2)*x)^2) returned x^2+2*x^2+2*sqrt(2)*x^2. The
2*x^2 term was not combined with the x^2 term to 3*x^2 because it
was not the canonical expair [[x^2,2]] but rather [[2*x^2,1]].

Thanks to Isuru Fernando for the bugreport.