[BUG] error when handling coefficients with fractions

Steps to reproduce:
1. URL encode `simplify('w/5-(2w+3)/2-3w/5')` to `simplify%28%27w%2F5-%282w%2B3%29%2F2-3w%2F5%27%29` using any URL encoder (I used https://www.urlencoder.io/).
2. Hit https://api.mathjs.org/v4/?expr=simplify%28%27w%2F5-%282w%2B3%29%2F2-3w%2F5%27%29 in the address bar in any web browser (I'm using Google Chrome).
3. Got output `w * -2 / 5 + (-(2 * w) - 3) / 2`.

Expected output:
$$\frac{w}{5}-\frac{2w+3}{2}-\frac{3w}{5} = -\frac75w-\frac32$$