Fix table header classes of has_many field

When displaying has_many resources in the show page, parent resource
name was used for child table header classes.

app/views/administrate/application/_collection.html.erb

@@ -25,7 +25,7 @@ to display a collection of resources in an HTML table.
         <th class="cell-label
         cell-label--<%= attr_type.html_class %>
         cell-label--<%= collection_presenter.ordered_html_class(attr_name) %>
-        cell-label--<%= "#{resource_name}_#{attr_name}" %>"
+        cell-label--<%= "#{collection_presenter.resource_name}_#{attr_name}" %>"
         scope="col"
         role="columnheader"
         aria-sort="<%= sort_order(collection_presenter.ordered_html_class(attr_name)) %>">

spec/features/show_page_spec.rb

@@ -99,6 +99,15 @@
     end
   end
 
+  it "adds has_many resource/attribute name to table headers" do
+    customer = create(:customer)
+    create_list(:order, 2, customer: customer)
+
+    visit admin_customer_path(customer)
+
+    expect(page).to have_css("th.cell-label--order_total_price")
+  end
+
   it "sorts each of the customer's orders" do
     customer = create(:customer)
     orders = create_list(:order, 4, customer: customer)