Area-spline incompatible with dashed regions

```
var chart = c3.generate({
data: {
columns: [
['data1', 30, 200, 100, 400, 150, 250],
['data2', 50, 20, 10, 40, 15, 25]
],
type:'spline',
regions: {
'data1': [{'start':1, 'end':2, 'style':'dashed'},{'start':3}], // currently 'dashed' style only
'data2': [{'end':3}]
}
}
});
```

this repro happens with splines in general, taken from the regions example and just add spline or area-spline as the type

refer to https://github.com/c3js/c3/issues/1684

Start with the supplied c3.generate reproduction and compare it with the regions example referenced in the issue, testing both spline and area-spline chart types. Done means dashed regions work correctly for these spline charts without breaking the other region styles.

Written by the indexing model from the issue text.