Make sure all middleware implement Service<Request<ReqBody>> and require Response = Response<ResBody>

[davidpdrsn]

For maximum flexibility in the future I think it makes sense to always implement Service like this:

impl<ReqBody, ResBody> Service<Request<ReqBody>> for Foo<S>
where
    S: Service<Request<ReqBody>, Response = Response<ResBody>>

That is they require the request type to be http::Request<_> and require the response of the inner service to be http::Response<_>.

If you only implement Service<R> then you don't have access to headers or extensions which might be necessary for implementing new features, same goes for the response. Case and point #156 which is a breaking change. That could have been avoided had ServeFile implemented Service<Request<_>>

For 0.2 I think we should go through all services fix this.