Fix:#821 Supports private phoenix_live_view session as tuple and map

[javierg]

This is a proposal solution for https://github.com/BeaconCMS/beacon/issues/821

What Is aw in my dev environment is that conn.private.phoenix_live_view extra session value is set as 3 items tuple, instead of a map, as expected. Not sure if this changed some where in the past and is actually expected. I can't figure it out clearly what was going on, and why the session is expected as a map, every test I did, returned as a tuple, not sure if this was a change on LiveView itself, I didn't research much from that side.

Hopefully you have time to review this and clarify if needed (I am using this branch for my working site as I need this custom error pages to work), If there is an alternative, I will appreciate it