Expose http::headers::{IterMut, ValuesMut}

It seems like these exports were not omitted intentionally.

The inability to name these types is giving me some grief in my work in tower-grpc where I'm trying to wrap HeaderMap in a wrapper that exposes an API in terms of gRPC metadata.

src/header/mod.rs

@@ -79,8 +79,10 @@ pub use self::map::{
     AsHeaderName,
     IntoHeaderName,
     Iter,
+    IterMut,
     Keys,
     Values,
+    ValuesMut,
     Drain,
     GetAll,
     Entry,