[Net\Browser] Check the order of options in the `Browser::post()` method

Reason: `CURLOPT_POSTFIELDS` implies setting `CURLOPT_POST` to `true`. Setting `CURLOPT_POST` to `true` implies setting request method to `POST`.
Needs checking if setting `CURLOPT_POSTFIELDS` after `CURLOPT_CUSTOMREQUEST` with `CURLOPT_POST` already set resets the request method.