fix(fetch): cancel request.body instead of source stream in request stream probe (#10702)

lib/adapters/fetch.js

@@ -66,18 +66,20 @@ const factory = (env) => {
     test(() => {
       let duplexAccessed = false;
 
-      const body = new ReadableStream();
+      const request = new Request(platform.origin, {
-
+        body: new ReadableStream(),
-      const hasContentType = new Request(platform.origin, {
-        body,
         method: 'POST',
         get duplex() {
           duplexAccessed = true;
           return 'half';
         },
-      }).headers.has('Content-Type');
+      });
 
-      body.cancel();
+      const hasContentType = request.headers.has('Content-Type');
+
+      if (request.body != null) {
+        request.body.cancel();
+      }
 
       return duplexAccessed && !hasContentType;
     });