fix rounding issue in sunbursts using _chart.equalRingSizes

[the3ver]

fixes #1661

[gordonwoodhull]

I noticed this problem when reviewing #1655 but then I forgot to say anything about it.

I prefer to fix the issue by being lenient in assertPortionsArray:

            if (Math.abs(portionsSum - 1) > dc.constants.NEGLIGIBLE_NUMBER) {

since this will come up for other relative ring sizes.

Took your tests, expanded to testing 2-27 equal rings, and applied the above fix for 3.2.1 and 4.0.3

Thanks @the3ver!