#65 keep creation date

[Laurent Roro]

When optimizing an image in a lossless way, it makes sense that the modification date changes but not the creation date.

context : I use optipng to optimize raytraced renders, and having access to the date is really useful (I can tell how long an image took to render, when it was rendered, which part had been re-rendered). Running optipng makes significant size improvement, but it messes up the dates.

thanks

Laurent