Source

File: wp-includes/Requests/Transport/cURL.php

public function stream_headers($handle, $headers) {
		// Why do we do this? cURL will send both the final response and any
		// interim responses, such as a 100 Continue. We don't need that.
		// (We may want to keep this somewhere just in case)
		if ($this->done_headers) {
			$this->headers = '';
			$this->done_headers = false;
		}
		$this->headers .= $headers;

		if ($headers === "\r\n") {
			$this->done_headers = true;
		}
		return strlen($headers);
	}