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
Amend mocks for TestHeaderHostUnmodified

Tell Nginx, which is reverse-proxying the request, to pass on the original
Host header.
Merge pull request #36 from alphagov/host_header

Implement TestHeaderHostUnmodified