[API] issue to reset zoom option properly

Hi,

I have an issue with zoom option .

On this example : https://c3js.org/samples/interaction_zoom.html

I have added this section of code
`setTimeout(function () {
chart.unzoom();
}, 5000);`

**Procedure to reproduce my issue :**

1. zoom on the chart
1. after 5 seconds zoom is reset
1. then try to zoom again, the zoom domain hasn't been really reset isn't it ?

How can we reset the zoom properly ?

Best reguards,
Leo.

Start by reproducing the behavior in the interaction_zoom.html sample using chart.unzoom() after the setTimeout shown in the issue. Trace the zoom reset entry point and verify that, when done, a later zoom begins from the chart's original domain rather than the previous zoom state.

Written by the indexing model from the issue text.