RoundUp can round a number that shouldn't be rounded

```go
d := decimal.RequireFromString("3.692").Mul(decimal.RequireFromString("1.25"))
fmt.Println(d)             // 4.615
fmt.Println(d.RoundUp(3))  // 4.616
```

at the same time

```go
d := decimal.RequireFromString("4.615")
fmt.Println(d.RoundUp(3))  // 4.615
```