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.

> >>> eq.as_coeff_Mul(rational=1)
> (1, -oo*x*cos(x)) # <---- should have been (-1, oo*x*cos(x))

I don't understand.  This commit doesn't touch as_coeff_Mul at all.

> This should be tested, too.

There are tests in this pr.  Did I miss something?

> If you make that change here, the same could be made in as_coeff_mul.

Do you mean as_coeff_Mul?