Mark AJAX requests as XMLHttpRequest

Hi,
during the initialization of webfm in the node edit form, the AJAX post requests to http://../webfm_js/ cause repeated calls of hook_init. This causes problems e.g. for the protected_node module which locks nodes for editing in its init function. To avoid such issues, jQuery started to mark ajax requests with an additional POST header:

// Set header so the called script knows that it's an XMLHttpRequest
xhr.setRequestHeader("X-Requested-With", "XMLHttpRequest");

The following patch adds the same header for the webfm ajax calls, so that modules can distinguish between real and AJAX POST requests. Please consider adding this, so that modules like protected_node are able to work together with webfm (see http://drupal.org/node/281584).