Log kramdown warnings if log level is WARN (#6522)

Merge pull request 6522

lib/jekyll/converters/markdown/kramdown_parser.rb

@@ -37,7 +37,12 @@ module Jekyll
         end
 
         def convert(content)
-          Kramdown::Document.new(content, @config).to_html
+          document = Kramdown::Document.new(content, @config)
+          html_output = document.to_html
+          document.warnings.each do |warning|
+            Jekyll.logger.warn "Kramdown warning:", warning
+          end
+          html_output
         end
 
         private

lib/jekyll/converters/smartypants.rb

@@ -29,7 +29,12 @@ module Jekyll
       end
 
       def convert(content)
-        Kramdown::Document.new(content, @config).to_html.chomp
+        document = Kramdown::Document.new(content, @config)
+        html_output = document.to_html.chomp
+        document.warnings.each do |warning|
+          Jekyll.logger.warn "Kramdown warning:", warning
+        end
+        html_output
       end
     end
   end

test/test_kramdown.rb

@@ -56,6 +56,12 @@ class TestKramdown < JekyllUnitTest
       assert_equal "<h1>Some Header</h1>", @markdown.convert("# Some Header #").strip
     end
 
+    should "should log kramdown warnings" do
+      allow_any_instance_of(Kramdown::Document).to receive(:warnings).and_return(["foo"])
+      expect(Jekyll.logger).to receive(:warn).with("Kramdown warning:", "foo")
+      @markdown.convert("Something")
+    end
+
     context "when asked to convert smart quotes" do
       should "convert" do
         assert_match(