Fix typing mismatch

If we're going to use typed header names and values, we'll have to
duplicate some of the string literals across the two. Not a big deal.

src/responders/vary.rs

@@ -1,12 +1,12 @@
 use axum_core::response::{IntoResponseParts, ResponseParts};
 use http::header::{HeaderValue, VARY};
 
-use crate::{extractors, headers, HxError};
+use crate::{extractors, HxError};
 
-const HX_REQUEST: HeaderValue = HeaderValue::from_static(headers::HX_REQUEST);
-const HX_TARGET: HeaderValue = HeaderValue::from_static(headers::HX_TARGET);
-const HX_TRIGGER: HeaderValue = HeaderValue::from_static(headers::HX_TRIGGER);
-const HX_TRIGGER_NAME: HeaderValue = HeaderValue::from_static(headers::HX_TRIGGER_NAME);
+const HX_REQUEST: HeaderValue = HeaderValue::from_static("hx-request");
+const HX_TARGET: HeaderValue = HeaderValue::from_static("hx-target");
+const HX_TRIGGER: HeaderValue = HeaderValue::from_static("hx-trigger");
+const HX_TRIGGER_NAME: HeaderValue = HeaderValue::from_static("hx-trigger-name");
 
 /// The `Vary: HX-Request` header.
 ///
@@ -125,10 +125,12 @@ impl extractors::HxTriggerName {
 
 #[cfg(test)]
 mod tests {
-    use super::*;
-    use axum::{routing::get, Router};
     use std::collections::hash_set::HashSet;
 
+    use axum::{routing::get, Router};
+
+    use super::*;
+
     #[tokio::test]
     async fn multiple_headers() {
         let app = Router::new().route("/", get(|| async { (VaryHxRequest, VaryHxTarget, "foo") }));