Substitute drupal_set_header() with drupal_add_http_header()

The file views-views-json-style-exhibit.tpl.php is using drupal_set_header(), a Drupal 6-only function. Using a function that is not defined in Drupal 7 causes this Internal Server Error. Substituting the D6-only function with drupal_add_http_header() solves the issue.

I did a search for drupal_set_header() in the views_datasource module, and apparently this is the only instance where the D6 function was being used.

Will attach patch next.