Injected into the Http service. This class contains application-wide HTTP defaults.

The default implementation provides headers which the Angular team believes to be useful.

class HttpDefaults {
 /**
  * The [HttpDefaultHeaders] object used by [Http] to add default headers
  * to requests.
  */
 HttpDefaultHeaders headers;

 /**
  * The default cache.  To enable caching application-wide, instantiate with a
  * [Cache] object.
  */
 var cache;

 /**
  * Constructor intended for DI.
  */
 HttpDefaults(HttpDefaultHeaders this.headers);
}