update: export HeaderName from crate root

Author: Velfi

src/lib.rs

@@ -185,7 +185,7 @@ mod extensions;
 pub use crate::error::{Error, Result};
 pub use crate::extensions::Extensions;
 #[doc(no_inline)]
-pub use crate::header::{HeaderMap, HeaderValue};
+pub use crate::header::{HeaderMap, HeaderName, HeaderValue};
 pub use crate::method::Method;
 pub use crate::request::Request;
 pub use crate::response::Response;