You will probably need to look at the sslverify argument and modify the plugin to use this. See the documentation http://codex.wordpress.org/HTTP_API
The ‘sslverify’ argument was added in Version 2.8 and will check to
see if the SSL certificate is valid (not self-signed, actually for the
site in the request) and will deny the response if it isn’t. If you
are requesting HTTPS and and know that the site is self-signed or is
invalided and are reasonably sure that it can be trusted, then set to
false.