json_encode Empty

Below my PHP code. It takes the data from the database, puts it in an array, and then passes it through a json_encode. However, it does not print anything, how to solve?

Note: Using print_r in the array it is possible to perfectly visualize the data

<?php        
    $user = "root";
    $password = "";
    $db = "angulardb";
    $host = "localhost";
    $con = mysqli_connect("localhost", $user, $password, $db);

    if (mysqli_connect_errno()){
      echo "Failed to connect to MySQL: " . mysqli_connect_error();
    }

    $usuario = mysqli_query($con, "SELECT *  FROM users");

    if (!$usuario) {
        echo "Não foi possível executar a consulta no banco de dados: " . mysql_error();
        exit;

    }

    $return = array();

    while ($dados = mysqli_fetch_assoc($usuario)) {
        $return[] = $dados;

    }
    header('Content-Type: application/json');   
    echo json_encode($return);

?>
Database:

CREATE TABLE IF NOT EXISTS 'users' (
  'id' int(6) NOT NULL AUTO_INCREMENT,
  'nome' varchar(50) NOT NULL,
  'email' varchar(50) NOT NULL,
  'pass' varchar(50) NOT NULL,
  PRIMARY KEY ('id')
) ENGINE=InnoDB  DEFAULT CHARSET=latin1 AUTO_INCREMENT=4 ;

INSERT INTO 'users' ('id', 'nome', 'email', 'pass') VALUES
(2, 'João Silva', '[email protected]', '123456'),
(3, 'Mario de Almeida', '[email protected]', '123456');