Correct the type of DispatchOptions["headers"]

According to core/util@parseHeaders, the current behavior is:

- By default, the entry type is a `string`.
- The entry type turns to a `string[]` when there are duplicated entries

This behavior is not as same as what `node:http` does currently;
therefore, `IncomingHttpHeaders` can't reflect our parsed data.

This commit attempts to reflect this parsing behavior.

Fixed nodejs#1892

Signed-off-by: pan93412 <pan93412@gmail.com>

types/dispatcher.d.ts

@@ -100,7 +100,7 @@ declare namespace Dispatcher {
     /** Default: `null` */
     body?: string | Buffer | Uint8Array | Readable | null | FormData;
     /** Default: `null` */
-    headers?: IncomingHttpHeaders | string[] | null;
+    headers?: Record<string, string | string[]> | null;
     /** Query string params to be embedded in the request URL. Default: `null` */
     query?: Record<string, any>;
     /** Whether the requests can be safely retried or not. If `false` the request won't be sent until all preceding requests in the pipeline have completed. Default: `true` if `method` is `HEAD` or `GET`. */