delegate `StaticFile#to_json` to `StaticFile#to_liquid` for a more (#6273)

Merge pull request 6273

lib/jekyll/static_file.rb

@@ -2,8 +2,12 @@
 
 module Jekyll
   class StaticFile
+    extend Forwardable
+
     attr_reader :relative_path, :extname, :name, :data
 
+    def_delegator :to_liquid, :to_json, :to_json
+
     class << self
       # The cache of last modification times [path] -> mtime.
       def mtimes

test/test_static_file.rb

@@ -176,5 +176,9 @@ class TestStaticFile < JekyllUnitTest
       }
       assert_equal expected, @static_file.to_liquid.to_h
     end
+
+    should "jsonify its liquid drop instead of itself" do
+      assert_equal @static_file.to_liquid.to_json, @static_file.to_json
+    end
   end
 end