Added example of non standard 'X-Requested-With' HTTP header.

This example is giving an example on how to add a non standard header,
and as well reminding the user to add a 'HTTP_' prefix, so the custom
header gets sent.

Author: Thomas Ploch

book/testing.rst

@@ -235,17 +235,19 @@ document::
         )
 
     The ``server`` array is the raw values that you'd expect to normally
-    find in the PHP `$_SERVER`_ superglobal. For example, to set the `Content-Type`
-    and `Referer` HTTP headers, you'd pass the following::
+    find in the PHP `$_SERVER`_ superglobal. For example, to set the `Content-Type`,
+    `Referer` and `X-Requested-With' HTTP headers, you'd pass the following (mind
+    the `HTTP_` prefix for non standard headers)::
 
         $client->request(
             'GET',
             '/demo/hello/Fabien',
             array(),
             array(),
             array(
-                'CONTENT_TYPE' => 'application/json',
-                'HTTP_REFERER' => '/foo/bar',
+                'CONTENT_TYPE'          => 'application/json',
+                'HTTP_REFERER'          => '/foo/bar',
+                'HTTP_X-Requested-With' => 'XMLHttpRequest'
             )
         );