Add support for the headline ingredient

Add headline to the list of supported ingredients. It contains valuable information and should be in the search index.

lib/alchemy-pg_search.rb

@@ -8,6 +8,7 @@ module Alchemy
   mattr_accessor :searchable_ingredients
   @@searchable_ingredients = {
     "Alchemy::Ingredients::Text": :value,
+    "Alchemy::Ingredients::Headline": :value,
     "Alchemy::Ingredients::Richtext": :stripped_body,
     "Alchemy::Ingredients::Picture": :caption,
   }

spec/models/ingredient_spec.rb

@@ -66,6 +66,13 @@
     it_behaves_like "a searchable content"
   end
 
+  describe Alchemy::Ingredients::Headline do
+    let(:ingredient) { create(:alchemy_ingredient_headline, value: "foo bar", element: element) }
+
+    it_behaves_like "it is searchable", field: :value
+    it_behaves_like "a searchable content"
+  end
+
   describe Alchemy::Ingredients::Richtext do
     let(:ingredient) { create(:alchemy_ingredient_richtext, value: "<b>foo</b> bar", element: element) }