Change from Scheme to IsSecure as authoritative.

#102
Scheme is not a field transmitted on the wire, the server must infer it based on if the transport layer uses SSL or not. This changes the feature abstraction so that IsSecure is the authoritative value and Scheme is derived from it (rather than the other way around like it is now).

This should not impact any developer code.

I have matching commits ready for WebListener, Helios, Kestrel, and TestHost.