Centering a single cell does not persist on re-editing a page.

Steps to reproduce:

1. Edit an article in eZ Publish 5.1 in IE8;
2. Create a table (e.g. 2x2) in eZOE;
3. Center a cell of your choice;
4. Publish article and re-edit. The cell is not centered, as the HTML to do so (e.g. <paragraph align="center">My content</paragraph>) is missing;