Add multiValueHeaders type to Request (#264)

Author: guirisnik

index.d.ts

@@ -154,6 +154,9 @@ export declare class Request {
   headers: {
     [key: string]: string | undefined;
   };
+  multiValueHeaders: {
+    [key: string]: string | undefined;
+  };
   rawHeaders?: {
     [key: string]: string | undefined;
   };