Flag cookies as secure with ignore case in ActionDispatch::SSL

> 5.2.5.  The Secure Attribute
>
> If the attribute-name **case-insensitively** matches the string "Secure",
> the user agent MUST append an attribute to the cookie-attribute-list
> with an attribute-name of Secure and an empty attribute-value.

cite: http://www.ietf.org/rfc/rfc6265.txt