`DecisionTreeClassifier` doesn't split node with `min_samples_split` examples

`DecisionTreeClassifier` doesn't split nodes with exactly `min_samples_split` examples due to this line 

```rust
        if n <= self.parameters().min_samples_split {
            return false;
        }
```
This should be 
```rust
        if n < self.parameters().min_samples_split {
            return false;
        }
```
as it is in `BaseTreeRegressor`.
