Set header in routes

I have set below route:

Route::get('robots.txt', 'MainController@robots');

In the controller I have this function:

public function robots() {
    header('Content-Type: text/plain');
    echo 'User-agent: *';
    echo PHP_EOL;
    echo 'Allow: /';
    echo PHP_EOL;
}

But when accessed via http://ift.tt/1sMWKqc It throws below error:

Exception 'ErrorException' with message 'Cannot modify header information - headers already sent by (output started at /home/domain/public_html/vendor/symfony/http-foundation/Response.php:1156)

So want to set the header header('Content-Type: text/plain'); in the routes.php to fix this problem. But my question is how?

Or is there a better approach?

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