protected function _prepareHeaders()
    {
        $headers = array();

        // Set the host header
        if (! isset($this->headers['host'])) {
            $host = $this->uri->getHost();

            // If the port is not default, add it
            if (! (($this->uri->getScheme() == 'http' && $this->uri->getPort() == 80) ||
                  ($this->uri->getScheme() == 'https' && $this->uri->getPort() == 443))) {
                $host .= ':' . $this->uri->getPort();
            }

            $headers[] = "Host: {$host}";
        }

        // Set the connection header
        if (! isset($this->headers['connection'])) {
            if (! $this->config['keepalive']) {
                $headers[] = "Connection: close";
            }
        }

        // Set the Accept-encoding header if not set - depending on whether
        // zlib is available or not.
        if (! isset($this->headers['accept-encoding'])) {
            if (function_exists('gzinflate')) {
                $headers[] = 'Accept-encoding: gzip, deflate';
            } else {
                $headers[] = 'Accept-encoding: identity';
            }
        }
        
        // Set the Content-Type header
        if ($this->method == self::POST &&
           (! isset($this->headers[strtolower(self::CONTENT_TYPE)]) && isset($this->enctype))) {

            $headers[] = self::CONTENT_TYPE . ': ' . $this->enctype;
        }
        
        // Set the user agent header
        if (! isset($this->headers['user-agent']) && isset($this->config['useragent'])) {
            $headers[] = "User-Agent: {$this->config['useragent']}";
        }

        // Set HTTP authentication if needed
        if (is_array($this->auth)) {
            $auth = self::encodeAuthHeader($this->auth['user'], $this->auth['password'], $this->auth['type']);
            $headers[] = "Authorization: {$auth}";
        }

        // Load cookies from cookie jar
        if (isset($this->cookiejar)) {
            $cookstr = $this->cookiejar->getMatchingCookies($this->uri,
                true, Zend_Http_CookieJar::COOKIE_STRING_CONCAT);

            if ($cookstr) {
                $headers[] = "Cookie: {$cookstr}";
            }
        }

        // Add all other user defined headers
        foreach ($this->headers as $header) {
            list($name, $value) = $header;
            if (is_array($value)) {
                $value = implode(', ', $value);
            }

            $headers[] = "$name: $value";
        }

        return $headers;
    }