Add empty body conversion from unit

[joshka]

This makes it possible to convert an http::Request with an empty body by
passing () to the body method. This is useful when writing sans-io
libraries where you want to create requests that are agnostic to the
specific reqwest implementation (async, blocking, wasm).

E.g.:

let http_request = http::Request::builder()
    .uri("http://localhost/")
    .body(())?;

let req = reqwest::Request::try_from(http_request)?;
reqwest::Client::new().execute(req).await?;

// or
let req = reqwest::blocking::Request::try_from(http_request)?;
reqwest::blocking::Client::new().execute(req)?;