roc_pivot - not affecting predict function

[jakwisn]

Post-processing function roc_pivot does affect only $y_hat of explainer and not the predict function. When dealing with new data it may be misleading. Therefore roc_pivot should also change predict function

Thanks @piotrpiatyszek for bringing it up

[jakwisn]

This is a good issue and it needs to be discussed in more depth as the changes for this to take effect are affecting DALEX. As for now, it will remain open.