Fix toFormat grouping by secondaryGroupSize when groupSize is 0

[chatman-media]

groupSize: 0 is supposed to disable grouping entirely (this was just fixed for the plain case in #407), but it doesn't work if secondaryGroupSize is also set. The two values get swapped internally for the Indian-style grouping logic, and the groupSize > 0 check that's meant to short-circuit grouping ends up looking at the secondary value instead.

BigNumber.config({ FORMAT: { groupSize: 0, secondaryGroupSize: 2 } });
new BigNumber(123456789).toFormat();   // '1,23,45,67,89' — should be '123456789'

Fixed by only doing the swap when the primary groupSize is actually greater than 0. Added tests next to the existing #407 cases, covering this combination.

[MikeMcl]

Well, nowhere is it stated that "groupSize: 0 is supposed to disable grouping entirely", so this is not a "fix" but a suggestion to alter the current behaviour, which is how this PR should have been framed.

The behaviour has always been that secondaryGroupSize always applies: groupSize is applied to the rightmost digits and secondaryGroupSize is applied repeatedly to the remaining digits on the left, if that is all the digits then so be it. If that grouping isn't wanted then just explicitly set secondaryGroupSize to 0, its default value.

Having said that, it may be a worthwhile change and I will look at it closely presently.

[lbesecker195]

Verified in toFormat: with groupSize: 0 and a secondaryGroupSize, the unconditional swap turned g1 into the secondary size and grouping ran anyway, undoing the #407 fix. Gating the swap on g1 > 0 leaves g1 = 0, so the g1 > 0 && len > 0 block is skipped and no grouping happens. Tests cover it. LGTM.