`@arcjet/bun`: silently does not read body when already read

When a user reads the body before passing `request` into `@arcjet/bun`, the SDK will receive a `""` body back. In Deno (and probably others), reading a used stream again will throw, which will cause SDKs to log an error and return an error decision. There is a `request.bodyUsed: boolean` field which could be used to tell users that `@arcjet/bun` cannot do work.