Change HeaderMap extractor to clone the headers (#698)

* Change `HeaderMap` extractor to clone the headers

* fix docs

* changelog

* inline variable

* also add changelog item to axum

* don't list types from axum in axum-core's changelog

* document that `HeaderMap::from_request` clones the headers

* fix typo

* a few more typos

axum-core/src/extract/rejection.rs

@@ -8,13 +8,6 @@ define_rejection! {
     pub struct BodyAlreadyExtracted;
 }
 
-define_rejection! {
-    #[status = INTERNAL_SERVER_ERROR]
-    #[body = "Headers taken by other extractor"]
-    /// Rejection used if the headers has been taken by another extractor.
-    pub struct HeadersAlreadyExtracted;
-}
-
 define_rejection! {
     #[status = INTERNAL_SERVER_ERROR]
     #[body = "Extensions taken by other extractor"]
@@ -47,7 +40,6 @@ composite_rejection! {
     /// [`Request<_>`]: http::Request
     pub enum RequestAlreadyExtracted {
         BodyAlreadyExtracted,
-        HeadersAlreadyExtracted,
         ExtensionsAlreadyExtracted,
     }
 }
@@ -79,7 +71,6 @@ composite_rejection! {
     ///
     /// Contains one variant for each way the [`http::request::Parts`] extractor can fail.
     pub enum RequestPartsAlreadyExtracted {
-        HeadersAlreadyExtracted,
         ExtensionsAlreadyExtracted,
     }
 }