Make out_file append option work without compression

For Ruby 2.7.x and 3.0.x, out_file append option without compression
does not work. The cause is the following Ruby language bug:

https://bugs.ruby-lang.org/issues/18388.

This commit is a workaround for this problem.

See #3569.

lib/fluent/plugin/out_file.rb

@@ -223,7 +223,12 @@ def write(chunk)
 
     def write_without_compression(path, chunk)
       File.open(path, "ab", @file_perm) do |f|
-        chunk.write_to(f)
+        if @append
+          content = chunk.read()
+          f.puts content
+        else
+          chunk.write_to(f)
+        end
       end
     end