Notice: Array to string conversion en drupal_send_headers()

Every time a image style is generated this notice is generated:
Notice: Array to string conversion en drupal_send_headers()

I've seen the code and I check in hook_file_download this code:

<?php
// Allow access. Another module implementing hook_file_download() may still
    // deny access to this image.
    if ($result == IMAGE_STYLE_ACCESS_ALLOW) {
      // Return file headers to allow access.
      return array(
        'Content-Type' => $info['mime_type'],
        'Content-Length' => $info['file_size'],
      );
    }
?>

This headers are not neccesary, image module do the same. If the headers are repeated drupal shows the warning.

I made a patch for this.

Thanks!