SfxItemSet::MergeValue changes some items from SFX_ITEM_DEFAULT state to
SFX_ITEM_SET which I think is a bug but this patch avoids the problem too.
The issue was: visible changes in some tables, after changing e.g.
borders spacing, because the cells had wrong SfxItemSet after the process.

Change-Id: I676b211e1a4a1d7341c385d63503aa740718ed5d