Fix destroy document specs

We were linking to the document url itself, which does not have a route
associated and so the specs fails

With this commit we are using the correct path to the destroy action of
the DocumentsController.

We are also using the referrer instead of a params[:from] attribute, as
it avoids having to pass an extra parameter, making the code prettier
and it works the same way

app/controllers/documents_controller.rb

@@ -11,7 +11,7 @@ class DocumentsController < ApplicationController
         else
           flash[:alert] = t "documents.actions.destroy.alert"
         end
-        redirect_to params[:from]
+        redirect_to request.referer
       end
       format.js do
         if @document.destroy