RedirectBack
============

Mark a page to return to. Redirect back to and forget about it later.

  # before_filter :login_required
  unless authenticated?
    return_here!                # sets session[:return_to]
    redirect_to login_url
  end

  # def login
  if user = User.auth(...)
    login! user
    redirect_back welcome_url   # prefers session[:return_to] over welcome_url
  end

Use hidden_return_to helper to embed a return_to param in forms.
Using a form params instead of session allows the user to have multiple
tabs open without clashing session values.

  <% form_for ... %>
    <%= hidden_return_to %>
    ...
  <% end %>