Correct the signature of setHeader

setHeader() takes either a string or a string array as it's second argument.

See https://nodejs.org/api/http.html#http_response_setheader_name_value

node/node.d.ts

@@ -502,7 +502,7 @@ declare module "http" {
         statusCode: number;
         statusMessage: string;
         headersSent: boolean;
-        setHeader(name: string, value: string): void;
+        setHeader(name: string, value: string | string[]): void;
         sendDate: boolean;
         getHeader(name: string): string;
         removeHeader(name: string): void;