Test for valid temporary file.

If ```$this->cookieFile = tempnam(sys_get_temp_dir(), 'curlcookie');``` fails (can't write to filesystem, etc.),  the problem isn't seen until ```saveCookiesToSession()``` when ```$this->cookieFile``` is a boolean null instead of a file name.  This adds a check to ```createCookieFile()``` to make sure the cookie file in the temporary directory can be created.