usage of spread operator with HeadersInit leads to unexpected behavior or errors

[Jesse Jackson]

Here's an example from json:

sift/mod.ts

Lines 227 to 252 in 424d5b7

 /** Converts an object literal to a JSON string and returns 
  * a Response with `application/json` as the `content-type`. 
  * 
  * @example 
  * ```js 
  * import { serve, json } from "https://deno.land/x/sift/mod.ts" 
  * 
  * serve({ 
  *  "/": () => json({ message: "hello world"}), 
  * }) 
  *``` 
  * */ 
 export function json( 
   // deno-lint-ignore no-explicit-any 
   jsobj: any, 
   init?: ResponseInit, 
 ): Response { 
   return new Response(JSON.stringify(jsobj) + "\n", { 
     statusText: init?.statusText ?? STATUS_TEXT.get(init?.status ?? Status.OK), 
     status: init?.status ?? Status.OK, 
     headers: { 
       "Content-Type": "application/json; charset=utf-8", 
       ...init?.headers, 
     }, 
   }); 
 } 

On line 249, init.headers is of type HeadersInit (which is Headers | string[][] | Record<string, string>). However, if it is not Record<string, string>, this usage of the spread operator will lead to unexpected behavior or errors.