Use image Urls for Emoji in GFM

Fixes #2617

app/helpers/application_helper.rb

@@ -156,4 +156,9 @@ module ApplicationHelper
     image_tag("authbuttons/#{file_name}",
               alt: "Sign in with #{provider.to_s.titleize}")
   end
+
+  def image_url(source)
+    root_url + path_to_image(source)
+  end
+  alias_method :url_to_image, :image_url
 end

lib/gitlab/markdown.rb

@@ -119,7 +119,7 @@ module Gitlab
       # parse emoji
       text.gsub!(EMOJI_PATTERN) do |match|
         if valid_emoji?($2)
-          image_tag("emoji/#{$2}.png", class: 'emoji', title: $1, alt: $1, size: "20x20")
+          image_tag(url_to_image("emoji/#{$2}.png"), class: 'emoji', title: $1, alt: $1, size: "20x20")
         else
           match
         end

spec/helpers/gitlab_markdown_helper_spec.rb

@@ -343,5 +343,9 @@ describe GitlabMarkdownHelper do
     it "should generate absolute urls for refs" do
       markdown("##{issue.id}").should include(project_issue_url(project, issue))
     end
+
+    it "should generate absolute urls for emoji" do
+      markdown(":smile:").should include("src=\"#{url_to_image("emoji/smile")}")
+    end
   end
 end