Ref 705216d: Synchronize ajax/xhr.js with master

```
   raw     gz Sizes
280869  83326 dist/jquery.js
 96035  33286 dist/jquery.min.js

   raw     gz Compared to 1.x-master @ d8576139c14fb777c24867d7f72d70d7334aabd1
  -597   -196 dist/jquery.js
   -82    -42 dist/jquery.min.js
```