= http: allow HTTP/1.0 request w/o `Host` header when building effective request Uri, fixes #348

There is one downside: for HTTP/1.0 requests without `Host` header the effective request Uri the application sees has no authority, since there is currently no way for *spray-can* to know it. (The Uri *does* contains a scheme though!).

In all other cases the application is guaranteed to get a Uri with a proper authority.
The only way around that would be that we allow the user to set a default authority...