This PR enables Edge API endpoints to receive a body.
This wasn't in the original PR, as thankfully pointed out by @zaiste in [this comment](https://github.com/vercel/next.js/pull/37481#discussion_r899440567) 🙏
## Related
- Fixes#37821
## Bug
- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`