Add ResponseHeader in pingora_http crate's prelude

For import consistency, put ResponseHeader into pingora_http crate's
prelude. This enables importing with `use pingora::prelude::*` to bring
ResponseHeader and RequestHeader together.

Fixes #551.

Includes-commit: e074c23
Replicated-from: #554

Author: PeppyDays

.bleep

@@ -1 +1 @@
-03479bcb7e6312754af45a43f6cdd173538a3cc2
+32a7a7b8037c590b5e845c24f12e74d02b4ead92

docs/user_guide/rate_limiter.md

@@ -20,7 +20,6 @@ Pingora provides a crate `pingora-limits` which provides a simple and easy to us
 ```rust
 use async_trait::async_trait;
 use once_cell::sync::Lazy;
-use pingora::http::ResponseHeader;
 use pingora::prelude::*;
 use pingora_limits::rate::Rate;
 use std::sync::Arc;
@@ -164,4 +163,4 @@ You can run the pre-made example code in the [`pingora-proxy` examples folder](h
 
 ```
 cargo run --example rate_limiter
-```
+```

pingora-http/src/lib.rs

@@ -43,6 +43,7 @@ pub use case_header_name::IntoCaseHeaderName;
 
 pub mod prelude {
     pub use crate::RequestHeader;
+    pub use crate::ResponseHeader;
 }
 
 /* an ordered header map to store the original case of each header name

pingora/examples/client.rs

@@ -13,7 +13,6 @@
 // limitations under the License.
 
 use pingora::{connectors::http::Connector, prelude::*};
-use pingora_http::RequestHeader;
 use regex::Regex;
 
 #[tokio::main]