Fix encoding errors when writing output file

lib/java_submission_tester/java_input_output_junit_test_runner.rb

@@ -76,7 +76,7 @@ module JavaSubmissionTester
         Tempfile.open("input-#{task.id}") do |input_file|
           input_file.write input.input.read()
           Tempfile.open("output-#{submission.id}") do |output_file|
-            output_file.write submission.output.read()
+            output_file.write submission.output.read().force_encoding("UTF-8")
             puts "input file:",input_file.read()
             puts "output file:",output_file.read()
             result = run_json_reporter(working_dir,