Don't construct a KleinRequest via adaptation

[wsanchez]

In #359, @glyph notes:

The real problem here though appears to be that we are constructing a KleinRequest via adaptation - apparently for no good reason. Rather than lie about the type of mapper, I think it would make more sense to replace KleinResource.render's invocation with a simple KleinRequest(request, mapper).

Technically what I'm proposing is an incompatible change, but given that KleinRequest(request) gives you a nonsense non-functional object without an attached mapper, I'm OK with a short deprecation cycle here.