mul_ ignores denominator factors

[tueda]

FORM 4.1 (Jun  9 2017, v4.1-20131025-354-g59fe368) 64-bits  Run: Fri Jun  9 18:58:56 2017
    S x;
    L F1 = mul_(2/3,5/7);          * 10/21
    L F2 = mul_(1/2+x/3,1/5+x/7);  * 1/10 + 29/210*x + 1/21*x^2
    P;
    .end

Time =       0.00 sec    Generated terms =          1
              F1         Terms in output =          1
                         Bytes used      =         20

Time =       0.00 sec    Generated terms =          3
              F2         Terms in output =          3
                         Bytes used      =         76

   F1 =
      10;

   F2 =
      21 + 29*x + 10*x^2;