Added non_secure_cookies to ActionDispatch::SSL

Allows you to whitelist particular cookies that you do not want to be forced over to secure-only when enabling `config.force_ssl`.

The use-case I encountered is that we have a Rails site that is entirely SSL (via `config.force_ssl`) but that also has another site that is majority readonly and mixed HTTP(S). From the secure application we wanted to manage a secure-only, HTTP-only token to identify the user, and at the same time manage a non-secure, non-HTTP only so that the partner site could know they were logged in from server- and client-side as required.

As the only thing we want to be non-secure is this one cookie it seemed sensible to leave `config.force_ssl` on and monkeypatch `ActionDispatch::SSL` to ignore the one cookie we are happy to be non-secure. That got me out of the corner I was in today but I thought it worth opening a PR to see if it was worth contributing upstream.