Deal with !empty($curl_options[CURLOPT_POST]) in browser.inc

Deal with this bit from DrupalWebTestCase.

if (!empty($curl_options[CURLOPT_POST])) {
  // This is a fix for the Curl library to prevent Expect: 100-continue
  // headers in POST requests, that may cause unexpected HTTP response
  // codes from some webservers (like lighttpd that returns a 417 error
  // code). It is done by setting an empty "Expect" header field that is
  // not overwritten by Curl.
  $curl_options[CURLOPT_HTTPHEADER][] = 'Expect:';
}