ST_3DClosestPoint with mixed dimensionality in input values

[nicklas]

In the example in #2034, there is input of 2d and 3d. ST_3DClosestPoint then by design assumes that the 2d value have a zero Z-value. But I guess it is more like undefined.

So I think that the most correct solution maybe is to handle the POINT(6 0) in ST_3DClosestPoint function as a infinite vertical line crossing POINT(6 0). Then the answer would be what strk expects in #2032.

I want feedback before I try to do anything about it.