Detect JSON Ajax requests by the response content-type (like is done with XML). Fixes #5709.

test/data/json.php

@@ -1,9 +1,13 @@
 <?php
 error_reporting(0);
+if ( $_REQUEST['header'] ) {
+	header("Content-type: application/json");
+}
+
 $json = $_REQUEST['json'];
 if($json) {
 	echo '[ {"name": "John", "age": 21}, {"name": "Peter", "age": 25 } ]';
 } else {
 	echo '{ "data": {"lang": "en", "length": 25} }';
 }
-?>
+?>