Fix globalQuantiles expression when getting legend data

[Jesus89]

From https://github.com/CartoDB/cartoframes/issues/724

Context ===

globalQuantiles expression when used in a ramp is not returning values properly. Here there are the cases in the internal editor. Open the console and run layer.viz.color.getLegendData() to debug:

• Case 1: Points example

Legend data:

[-Infinity, Infinity]
[-Infinity, 0.001]
[0.001, 2]
[2, 4]
[4, Infinity]

• Case 2: Lines example

Legend data:

[-Infinity, Infinity]
[-Infinity, Infinity]
[-Infinity, Infinity]
[-Infinity, 1]
[1, Infinity]

[Jesus89]

I would be nice that instead of Infinity or -Infinity, it returned the global min/max to have more meaningful legends.

[elenatorro]

I like this idea, I am thinking about how to implement it for the rest of classifiers.