Fix for nice() at certain scales.

[danielyule]

The fix for #9 was causing me some problems with scale. I was getting the following behavior, which I'm pretty sure isn't expected:

const scale = d3.scaleLinear().domain([0, 14.1]).nice(5);
scale.domain(); // [0, 15]
scale.ticks(5); // [0, 2, 4, 6, 8, 10, 12, 14]

I was essentially having the reverse problem to the other users in in the issue. When stepping through, nice() was selecting a step size of 2 in its first calculation, then going back to 5 for its second (and thus not updating the domain). However, ticks() was calculating 2 as its step size, and therefore only going up to 14.

These are not the only numbers where I was seeing this problem, I just picked these ones in particular for an example.

The fix here updates the domain twice, instead of only updating the step size twice. I think it fixes my problem, without adversely affecting any other situations. In the example I gave, we now have

const scale = d3.scaleLinear().domain([0, 15]).nice(5);
scale.domain(); // [0, 20]
scale.ticks(5); // [0, 5, 10, 15, 20]

I think under ideal circumstances we'd have,

scale.domain([0, 15]).ticks(5) // [0, 5, 10, 15]

But changing that would require changing the tick error constants, and I'm guessing there's a reason they are the way they are.

[mbostock]

I’ve revised your implementation slightly, but it should be functionally equivalent. Seems like a reasonable fix to me. Thank you!