fix minor bug in write_submission_binary_file method on assignment export performer
fix minor bug in write_submission_binary_file method on assignment export performer

mkoon/gradecraft-development,UM-USElab/gradecraft-development,UM-USElab/gradecraft-development,UM-USElab/gradecraft-development,mkoon/gradecraft-development,UM-USElab/gradecraft-development,mkoon/gradecraft-development