By contract the first argument of the `fields_get` method is supposed to
be a list. Before this fix, `web_timeline` would call `fields_get` with
a string instead of a list.
Fortunately in case only 1 field is being grouped, this worked as Odoo
does an `x in y` comparison in its `fields_get` implementation, which
does pass in a simple `"project_id" in "project_id"` case.
But that call remains invalid and can break when `fields_get` has been
tweaked by other modules.
