Added the twitter:description tag (#464)

Merge pull request 464

Author: amirmasoudabdol

lib/template.html

@@ -18,6 +18,7 @@
 {% if seo_tag.description %}
   <meta name="description" content="{{ seo_tag.description }}" />
   <meta property="og:description" content="{{ seo_tag.description }}" />
+  <meta property="twitter:description" content="{{ seo_tag.description }}" />
 {% endif %}
 
 {% if site.url %}

spec/jekyll_seo_tag_integration_spec.rb

@@ -124,6 +124,7 @@
     it "uses the page description" do
       expect(output).to match(%r!<meta name="description" content="foo" />!)
       expect(output).to match(%r!<meta property="og:description" content="foo" />!)
+      expect(output).to match(%r!<meta property="twitter:description" content="foo" />!)
     end
   end
 
@@ -133,6 +134,7 @@
     it "uses the page excerpt when no page description exists" do
       expect(output).to match(%r!<meta name="description" content="foo" />!)
       expect(output).to match(%r!<meta property="og:description" content="foo" />!)
+      expect(output).to match(%r!<meta property="twitter:description" content="foo" />!)
     end
   end
 
@@ -142,6 +144,7 @@
     it "uses the site description when no page description nor excerpt exist" do
       expect(output).to match(%r!<meta name="description" content="foo" />!)
       expect(output).to match(%r!<meta property="og:description" content="foo" />!)
+      expect(output).to match(%r!<meta property="twitter:description" content="foo" />!)
     end
   end