Donut chart with 100% is not completely closed

* **C3 version**:
- v0.6.8
* **D3 version**:
- 5
* **Browser**:
- Firefox Latest
* **OS**:
- Ubuntu 18.04

When one serie is 100% in a donut chart - the charts has a small "gap" at the top.

```
var chart = c3.generate({
data: {
columns: [
['data1', 100],
['data2', 0],
],
type : 'donut'
},
donut: {
title: "Iris Petal Width"
}
});
```

yields:


But:

```
var chart = c3.generate({
data: {
columns: [
['data1', 0],
['data2', 100],
],
type : 'donut'
},
donut: {
title: "Iris Petal Width"
}
});
```

yields:


Start by reproducing the provided c3.generate donut examples with D3 5 in Firefox, comparing the cases where data1 or data2 is 100%. Inspect the donut rendering entry point to find why one 100% series leaves a gap; done means a 100% donut is completely closed in both examples.

Written by the indexing model from the issue text.