httr2 0.2.3

• New example_url() to launch a local server, making tests and examples
  more robust.

• New throttle_status() to make it a little easier to verify what's happening
  with throttling.

• req_oauth_refresh() now respects the refresh_token for caching
  (@mgirlich, #178).

• req_perform() now always sleeps before a request, rather than after it.
  It also gains an error_call argument and communicates more clearly
  where the error occurred (@mgirlich, #187).

• req_url_path() and req_url_path_append() can now handle NULL or empty
  ... and the elements of ... can also have length > 1 (@mgirlich, #177).

• sys_sleep() (used by req_retry() and req_throttle()) gains a progress
  bar (#202).