add support for categories frontmatter

lib/jekyll-feed/feed.xml

@@ -71,9 +71,9 @@
         {% endif %}
       </author>
 
-      {% if post.category %}
-        <category term="{{ post.category | xml_escape }}" />
-      {% endif %}
+      {% for category in post.categories %}
+        <category term="{{ category | xml_escape }}" />
+      {% endfor %}
 
       {% for tag in post.tags %}
         <category term="{{ tag | xml_escape }}" />

spec/fixtures/_posts/2015-05-12-liquid.md

@@ -1,4 +1,5 @@
 ---
+categories: [first, second, third]
 ---
 
 {% capture liquidstring %}

spec/jekyll-feed_spec.rb

@@ -303,6 +303,19 @@
     end
   end
 
+  context "with 'categories' or 'category' or 'tags' key in the front matter" do
+    let(:feed) { RSS::Parser.parse(contents) }
+    let(:entry_with_single_category) { feed.items.find { |i| i.title.content == "March The Second" } }
+    let(:entry_with_multiple_categories) { feed.items.find { |i| i.title.content == "Liquid" } }
+    let(:entry_with_multiple_categories_and_tags) { feed.items.find { |i| i.title.content == "March The Fourth" } }
+
+    it "generates the feed correctly" do
+      expect(entry_with_single_category.categories.map(&:term)).to eql(%w(news))
+      expect(entry_with_multiple_categories.categories.map(&:term)).to eql(%w(first second third))
+      expect(entry_with_multiple_categories_and_tags.categories.map(&:term)).to eql(["updates", "jekyll", "\"/><VADER>"])
+    end
+  end
+
   context "changing the file path via collection meta" do
     let(:overrides) do
       {