Add OCS API header per default

Relieve @PVince from having to write it all the time ;-)

core/js/oc-requesttoken.js

@@ -1,3 +1,4 @@
 $(document).on('ajaxSend',function(elm, xhr) {
 	xhr.setRequestHeader('requesttoken', oc_requesttoken);
-});
+	xhr.setRequestHeader('OCS-APIREQUEST', 'true');
+});