add delete method

superagent/superagent-tests.ts

@@ -52,6 +52,10 @@ request
   .del('/user/1')
   .end(callback);
 
+request
+  .delete('/user/1')
+  .end(callback);
+
 request('/search')
   .end(callback);
 

superagent/superagent.d.ts

@@ -26,6 +26,7 @@ declare module "superagent" {
       put(url: string, callback?: CallbackHandler): Req;
       head(url: string, callback?: CallbackHandler): Req;
       del(url: string, callback?: CallbackHandler): Req;
+      delete(url: string, callback?: CallbackHandler): Req;
       options(url: string, callback?: CallbackHandler): Req;
       trace(url: string, callback?: CallbackHandler): Req;
       copy(url: string, callback?: CallbackHandler): Req;