Avoid 'Cannot redirect to a parameter hash' error

Rails 4 introduced a [protection against passing parameters hash into
`redirect_to`](rails/rails#16170) as a security
measure.

In some circumstances the `session[:document_filters]` could contain a
paramters hash (I suspect this may only be the case for a session which
was serialized into someones session cookie in the pre-rails-4 whitehall
app and deserialized in this release).

Adding this `.to_h` forces the hash to always be converted to a ruby
hash.

It's safe to pass these parameters to redirect_to because they are all
features which are intended to be controlled by user input. The redirect
is just a feature to ensure consistency of urls.

app/controllers/admin/edition_workflow_controller.rb

@@ -214,6 +214,6 @@ def action_name_as_human_interaction(action_name)
   end
 
   def session_filters
-    session[:document_filters] || {}
+    (session[:document_filters] || {}).to_h
   end
 end

app/controllers/admin/editions_controller.rb

@@ -305,7 +305,7 @@ def default_filters
   end
 
   def session_filters
-    session[:document_filters] || {}
+    (session[:document_filters] || {}).to_h
   end
 
   def params_filters