MDL-72080 javascript: explicit Content-Type in service responses.

This fixes various external methods that initialize page output
manually, e.g. `core_get_fragment`, which can mess with third party
proxies that modify data in transit because of the current default
response type header (text/html).

Co-authored-by: Mario Torres

lib/ajax/service.php

@@ -115,4 +115,5 @@
     header('Accept-Ranges: none');
 }
 
+header('Content-Type: application/json; charset=utf-8');
 echo json_encode($responses);