Add rational kwarg for Mul.as_coeff_mul, like for as_coeff_Mul

this fix the printing problem, mentioned [here](https://github.com/sympy/sympy/issues/8398#issuecomment-62069573).  The problem was introduced in #8305.

This should be tested, too.

If I wrote that condition with the two `not` I don't know why. I think it's clearer as `if args[0].is_Number and (not rational or args[0].is_Rational):`. If you make that change here, the same could be made in `as_coeff_mul`.