Do not deactivate-mark if region active when exchange-point-and-mark

From the emacs official documentation:

> The command `C-x C-x' (`exchange-point-and-mark') exchanges the
> positions of point and the mark.  `C-x C-x' is useful when you are
> satisfied with the position of point but want to move the other end of
> the region (where the mark is).

We should preserve this feature with `prelude-exchange-point-and-mark`,
when the function is called with an active region, I think, we should
not deactivate the mark.