fix: pass request body to cache handler

[AlCalzone]

Fixes the 2nd half of https://github.com/sindresorhus/got/issues/2138, see my comment

Checklist

• [x] I have read the documentation.
• [x] I have included a pull request description of my changes.
• [x] I have included some tests.
• [ ] If it's a new feature, I have included documentation updates in both the README and the types.

[sindresorhus]

// @alphmth

[AlCalzone]

@sindresorhus anything else I need to do here?