types: align RequestInit.body type with lib.dom.ts (nodejs#2956)

Allow null.

Author: jdufresne

test/types/fetch.test-d.ts

@@ -31,6 +31,7 @@ const requestInit2: RequestInit = {
 const requestInit3: RequestInit = {}
 // Test assignment. See https://github.com/whatwg/fetch/issues/1445
 requestInit3.credentials = 'include'
+const requestInit4: RequestInit = { body: null }
 
 declare const request: Request
 declare const headers: Headers

types/fetch.d.ts

@@ -122,7 +122,7 @@ export interface RequestInit {
   method?: string
   keepalive?: boolean
   headers?: HeadersInit
-  body?: BodyInit
+  body?: BodyInit | null
   redirect?: RequestRedirect
   integrity?: string
   signal?: AbortSignal | null