resolves #149 merge category into categories; tag into tags (#157)

lib/jekyll-asciidoc/integrator.rb

@@ -79,6 +79,13 @@ def integrate document, collection_name = nil
           data.update adoc_data
         end
 
+        { 'category' => 'categories', 'tag' => 'tags' }.each do |sole_key, coll_key|
+          if (sole_val = data.delete sole_key) &&
+              !((coll_val = (data[coll_key] ||= [])).include? sole_val)
+            coll_val << sole_val 
+          end
+        end
+
         case data['layout']
         when nil
           document.content = %(#{StandaloneOptionLine}#{document.content}) unless data.key? 'layout'

spec/fixtures/with_posts/_posts/2016-02-02-post-with-singular-vars.adoc

@@ -0,0 +1,7 @@
+= Post With Categories
+:page-category: node
+:page-categories: [code, javascript]
+:page-tag: tip
+:page-tags: [syntax, beginner]
+
+Lorem ipsum.

spec/jekyll-asciidoc_spec.rb

@@ -759,6 +759,17 @@
       end
     end
 
+    it 'should merge singular variables with collection variables' do
+      post = find_post '2016-02-02-post-with-singular-vars.adoc'
+      expect(post).not_to be_nil
+      expect(post.data['categories']).to eql(['code', 'javascript', 'node'])
+      expect(post.data['tags']).to eql(['syntax', 'beginner', 'tip'])
+      if ::Jekyll::MIN_VERSION_3
+        file = output_file 'code/javascript/node/2016/02/02/post-with-singular-vars.html'
+        expect(::File).to exist(file)
+      end
+    end
+
     it 'should convert revdate to local Time object and use it as date of post' do
       # NOTE Time.parse without time zone assumes time zone of site
       date = ::Time.parse('2016-06-15 10:30:00')