fix: do not use frozen object for headers (#334)

Sometimes the headers are mutated by the environment the code is run in...

resolves storacha/w3up#1254

packages/transport/src/car/response.js

@@ -34,7 +34,7 @@ export const encode = (message, options) => {
   })
 
   return {
-    headers: HEADERS,
+    headers: { ...HEADERS },
     body,
   }
 }