fix: avoid panic when translating from hyperium

Before it always paniced on new headers while trying to blankly unwrap
the return of a hashmap insert.

This makes it:
  a. no longer unwrap at all, which was incorrect, and
  b. use append for duplicates

The opposite direction also uses hyperium's append, but (I think) this
was written before that same functionality existed in http-types.

src/hyperium_http.rs

@@ -61,7 +61,7 @@ fn hyperium_headers_to_headers(hyperium_headers: http::HeaderMap, headers: &mut
         if let Some(name) = name {
             let name = name.as_str().as_bytes().to_owned();
             let name = unsafe { HeaderName::from_bytes_unchecked(name) };
-            headers.insert(name, value).unwrap();
+            headers.append(name, value);
         }
     }
 }