It's probably simpler to delete immediately rather that deferring work. Save an ArrayList allocation!

FYI: I think you want to document that Retrofit doesn't support multiply-valued headers.
```
  public void foo(@Header("Cookie") String sessionCookie, @Header("Cookie") String advertisingCookie);
```
(This code looks like it's probably broken for this case. Which is okay. Just document that Retrofit supports one value per header and you're good.)