fix: not sending correctly application/x-www-form-urlencoded

[DanielRamosAcosta]

Issue

Fixes #27

Details

For now, this PR only adds a failing test. If you think the test is ok, I can try fixing the necessary code.

CheckList

• [ ] PR starts with #27 .
• [ ] Has been tested (where required) before merge to main.

[cmorten]

Closing for now as this doesn’t seem right - see https://github.com/asos-craigmorten/superoak/issues/27#issuecomment-1002550664