ActiveResource should allow SSL verification

As shipped right now, all ARes clients are subject to man-in-the-middle attacks by any server claiming to be a trusted API provider. We're using ActiveResource in some security-sensitive internal applications, and the lack of SSL certificate verification is worrisome.

I understand that configuring OpenSSL to properly verify all certificates in an application may be too high a barrier for some applications, but it would be preferable to at least have the option to override the default 'trust anyone' setting made in active_resource/connection.rb.

I've attached a simple patch against the current edge tree as an example; the particular configuration variable name is less important to me than the ability to turn on certificate verification in cases where the certificate infrastructure allows it.