Implement TestHeaderHostUnmodified

Verify that the CDN doesn't modify the Host header of the original request.

NB: Go doesn't present `Host` in `http.Header`. It gets stripped off and set
as a field of `http.Request`, as seen here:

https://code.google.com/p/go/source/browse/src/pkg/net/http/request.go?name=go1.2#60
https://code.google.com/p/go/source/browse/src/pkg/net/http/request.go?name=go1.2#576
This asserts/documents the `http.Request` behaviour described in the commit message. Is that useful to do?