Add tests for privates methods in view's helpers

Test that using private methods in `options_from_collection_for_select`
is deprecated.

Make the unused `secret` paramether in the `Post` Struct private to use
it in the test.

actionview/test/template/form_options_helper_test.rb

@@ -21,7 +21,12 @@ class FormOptionsHelperTest < ActionView::TestCase
   tests ActionView::Helpers::FormOptionsHelper
 
   silence_warnings do
-    Post        = Struct.new("Post", :title, :author_name, :body, :secret, :written_on, :category, :origin, :allow_comments)
+    Post        = Struct.new("Post", :title, :author_name, :body, :written_on, :category, :origin, :allow_comments) do
+                    private
+                      def secret
+                        "This is super secret: #{author_name} is not the real author of #{title}"
+                      end
+                  end
     Continent   = Struct.new("Continent", :continent_name, :countries)
     Country     = Struct.new("Country", :country_id, :country_name)
     Firm        = Struct.new("Firm", :time_zone)
@@ -68,6 +73,14 @@ def test_collection_options
     )
   end
 
+  def test_collection_options_with_private_value_method
+    assert_deprecated(/Using private methods in view's helpers is deprecated/) {  options_from_collection_for_select(dummy_posts, "secret", "title") }
+  end
+
+  def test_collection_options_with_private_text_method
+    assert_deprecated(/Using private methods in view's helpers is deprecated/) {  options_from_collection_for_select(dummy_posts, "author_name", "secret") }
+  end
+
   def test_collection_options_with_preselected_value
     assert_dom_equal(
       "<option value=\"&lt;Abe&gt;\">&lt;Abe&gt; went home</option>\n<option value=\"Babe\" selected=\"selected\">Babe went home</option>\n<option value=\"Cabe\">Cabe went home</option>",