A few small refactorings (#655)

* Simplify json content-type check

* Import HeaderMap from http instead of from headers

The headers crate is an optional dependency and its HeaderMap re-export
is `#[doc(hidden)]`.

* Use headers re-export in axum in examples

examples/jwt/src/main.rs

@@ -9,12 +9,12 @@
 use axum::{
     async_trait,
     extract::{FromRequest, RequestParts, TypedHeader},
+    headers::{authorization::Bearer, Authorization},
     http::StatusCode,
     response::{IntoResponse, Response},
     routing::{get, post},
     Json, Router,
 };
-use headers::{authorization::Bearer, Authorization};
 use jsonwebtoken::{decode, encode, DecodingKey, EncodingKey, Header, Validation};
 use once_cell::sync::Lazy;
 use serde::{Deserialize, Serialize};