Fix contour labels that required "thousands" suffixes

[etpinard]

fixes #2755, by setting exponentformat to 'B' in contour label mock axis object- to prevent faulty fallback in Axes.prepTicks and Axis.tickText.

cc @alexcjohnson

[alexcjohnson]

Nice, I was hoping that one would be easy. Curious though, how did contour_edge_cases not run into this problem?

At some point we should make this exponentformat configurable, but that's a feature rather than a bug.

💃

[etpinard]

Those undefined text pieces reported in #2755 came from this block:

plotly.js/src/plots/cartesian/axes.js

Lines 1338 to 1340 in 194ba1d

	else if(isSIFormat(exponentFormat)) {
	v += SIPREFIXES[exponent / 3 + 5];
	}

where exponent := 4 (4 for thousands here I think) which isn't covered in contour_edge_cases mock.