test(header): assert HeaderMap types are Sync in test_bounds

The check_bounds helper used a duplicated Send bound (Send + Send), so
the intended Sync assertion was never enforced and the test silently
checked Send only. Use Send + Sync so the test fails to compile if any
of the HeaderMap iterator or entry types ever stops being Sync.

Refs https://github.com/hyperium/http/issues/783

src/header/map.rs

@@ -4026,7 +4026,7 @@ mod as_header_name {
 
 #[test]
 fn test_bounds() {
-    fn check_bounds<T: Send + Send>() {}
+    fn check_bounds<T: Send + Sync>() {}
 
     check_bounds::<HeaderMap<()>>();
     check_bounds::<Iter<'static, ()>>();