Please re-export the `http` crate.

## Motivations

reqwest-middleware uses `http::Extensions`  and `http::Error` in its public API.

Atm, users have to add exact the same version of `http` to their `Cargo.toml` when they want to use `reqwest-middleware`.

It is best practice to re-export either the whole `http` crate or perhaps only the affected types.




reqwest-middleware-0.5.1