form_for fails on Spree with "undefined method `human_attribute_name' for NilClass:Class" for any field

[rterbush]

Trying to use your helpers on Spree. Finding that it works great on some forms, but on following login form, it fails with the above error. The form works fine if I use form_for_without_foundation.

<%= form_for(:spree_user, :url => spree.spree_user_session_path) do |f| %>
  <div id="password-credentials">
    <p>
      <%= f.email_field :email %>
    </p>
    <p>
      <%= f.password_field :password, :class => 'title', :tabindex => 2 %>
    </p>
  </div>
  <p>
    <label>
      <%= f.check_box :remember_me %>
    </label>
  </p>

  <p><%= f.submit Spree.t(:login), :class => 'tiny round button', :tabindex => 3 %></p>
<% end %>

[rterbush]

This appears to be an issue in spree_auth_devise that I will provide a PR for.