Fix replacing top slabs giving wrong/invalid data values

This issue was reported on [reddit](http://www.reddit.com/r/MCEdit/comments/2kxj7j/error_replace_tool_with_top_slab_not_working_right/). The fix was to remove retain data value logic from the fill blocks method which I think isn't very useful. If this functionality is wanted I think a better implementation is needed(something using block states) and an explicit option.