fix(ajax): ensure XHR send is being called

I am really not sure how we did not catch this. I suspect jasmine-ajax does not check to
see if send as been called on a given XHR. Really strange

Author: benlesh

Diff for: src/observable/dom/ajax.ts

@@ -198,6 +198,8 @@ export class AjaxSubscriber<T> extends Subscriber<Event> {
     if (result === errorObject) {
       return this.error(errorObject.e);
     }
+
+    xhr.send();
   }
 
   private createXHR(): XMLHttpRequest {