Fix list/grid view display when POST method is used by page providers

Since NXP-29126, it's possible to use the POST method (instead of GET) to execute page providers.

Once it's configured, the listing is incorrectly displayed with duplicates, because of incorrect values for the "offset" attributes

Steps to reproduce:

1. configure Nuxeo with this contribution
2. start Nuxeo
3. load 50 items in a folder
4. sort by title
5. scroll down
6. observe that the 41st item in the list is the same as the first one. Same thing for 42nd/2nd, etc

Excepted behavior:
All items in the grid/view list must be displayed correctly when I scroll among the results