Fix usage of multi-selection=false on nuxeo-data-table

The default value of "multiSelection" in the "nuxeo-data-table" element (iron-data-table.js) is set to true.

https://github.com/nuxeo/nuxeo-elements/blob/maintenance-3.1.x/ui/nuxeo-data-table/iron-data-table.js#L495

Because of that, it's not possible to disable the multi-selection on a data-table.

The code below

<nuxeo-data-table
   multi-selection="false"

will have no effect.

Expected behavior: there is a solution to disable the multi-selection

Note : changing the property multiSelection to false on the element "nuxeo-data-table" allows to switch to single selection. However doing that brings 2 issues :

• the default behavior cannot be changed for our customers, so I guess this is not an option to change property multiSelection to false
• I tested to switch to multi-selection by adding the attribut multi-selection to "nuxeo-data-table" but it did not work. Code I use

 <nuxeo-data-table
   multi-selection
   ...