absolute_url should not mangle URL if called more than once (#5789)

Merge pull request 5789

lib/jekyll/filters/url_filters.rb

@@ -10,6 +10,7 @@ module Jekyll
       # Returns the absolute URL as a String.
       def absolute_url(input)
         return if input.nil?
+        return input if Addressable::URI.parse(input).absolute?
         site = @context.registers[:site]
         return relative_url(input).to_s if site.config["url"].nil?
         Addressable::URI.parse(site.config["url"] + relative_url(input)).normalize.to_s

test/test_filters.rb

@@ -402,6 +402,11 @@ class TestFilters < JekyllUnitTest
         })
         assert_equal "http://xn--mlaut-jva.example.org/", filter.absolute_url(page_url)
       end
+
+      should "not modify an absolute URL" do
+        page_url = "http://example.com/"
+        assert_equal "http://example.com/", @filter.absolute_url(page_url)
+      end
     end
 
     context "relative_url filter" do