Fix bug in body buffering when body is insta-aborted

The preexisting code tried to reject the promise and attach failure
metadata before it was actually initialized. It's now deferred. This is
a tricky edge case to hit and didn't seem to come up before the recent
tightening of body & abort handling.

Author: pimterry

src/util/buffer-utils.ts

@@ -93,7 +93,7 @@ export const streamToBuffer = (input: stream.Readable, maxSize = MAX_BUFFER_SIZE
 
             // If stream has already finished/aborted, resolve accordingly immediately:
             if (input.readableEnded) return resolve(Buffer.from([]));
-            if (input.readableAborted) return failWithAbortError();
+            if (input.readableAborted) return setImmediate(failWithAbortError);
 
             let currentSize = 0;
             const onData = (d: Buffer) => {