Fixes from review feedback

lib/httpRequest.ts

@@ -7,6 +7,7 @@ import { BodyInit as NodeBodyInit } from "node-fetch";
 
 /**
  * A value that can be used as an HTTP request body.
+ * Allows any fetch request body type or a Node.js readable stream.
  */
 export type HttpRequestBody = RequestInit["body"] | NodeBodyInit;
 

lib/inMemoryHttpResponse.ts

@@ -35,7 +35,7 @@ export class InMemoryHttpResponse implements HttpResponse {
     return Promise.resolve(this._deserializedBody);
   }
 
-  get readableStreamBody(): ReadableStream | NodeJS.ReadableStream {
+  get readableStreamBody(): ReadableStream | NodeJS.ReadableStream | null {
     // TODO: create appropriate stream type based on environment
     throw new Error("Stream body not supported in InMemoryHttpResponse");
   }