Fixed an issue with the baseline in rem function

Consider the following code snippet:

```scss
.test {
  padding: rem(10px 20px, 15px);
}
```

As of today, it yields `1rem 2rem`, because if the first argument is a list, the baseline is not correctly passed in the recursive call, leading to a calculation with the default baseline.

With the fix, it should now yields `0.66667rem 1.33333rem`.
Merge pull request #5 from HugoGiraudel/patch-1

Fixed an issue with the baseline in rem function