backdrop_http_request() now respects HTTP 429 "Too Many Requests" response code

Versions of Backdrop prior to 1.27.2 were treating HTTP 429 response codes generically as a 400 response when using backdrop_http_request(). It is now possible to explicitly distinguish 429 responses, instead of falling back to their "base" response code 400.

Versions prior to 1.27.2:

$response = backdrop_http_request('http://example.com/api/request');
if ($response->code == 400) {
  // Handle Bad Request errors as well as Too Many Request errors.
}

After 1.27.2:

$response = backdrop_http_request('http://example.com/api/request');
if ($response->code == 429) {
  // Handle Too Many Request errors.
}
elseif ($response->code == 400) {
  // Handle Bad Request errors.
}