docs(body) fix doctest failing due to missing features

The doctest uses `hyper::Client`, but that is not available unless these features are enabled.
This commit adds an attribute to check for those and allows `cargo test` with no arguments to pass again.

fixes #2687.

src/body/to_bytes.rs

@@ -17,6 +17,7 @@ use super::HttpBody;
 /// # Example
 ///
 /// ```
+/// # #[cfg(all(feature = "client", any(feature = "http1", feature = "http2")))]
 /// # async fn doc() -> hyper::Result<()> {
 /// use hyper::{body::HttpBody};
 ///