httr2 1.0.3

• jwt_encode_hmac() now calls correct underlying function
  jose::jwt_encode_hmac() and has correct default size parameter value
  (@denskh, #508).

• req_cache() now prunes cache before checking if a given key exists,
  eliminating the occassional error about reading from an invalid RDS file.
  It also no longer tests for existence then later reads the cache, avoiding
  potential race conditions.

• New req_perform_promise() creates a promises::promise so a request can
  run in the background (#501, @gergness).

• req_perform_parallel() now respects error handling in req_error().