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.

I think you should process -oo first (change order of conditions)

```python
>>> eq.as_coeff_mul(rational=1)
(-1, (oo, x, cos(x)))
>>> eq.as_coeff_Mul(rational=1)
(1, -oo*x*cos(x)) # <---- should have been (-1, oo*x*cos(x))
```