Fixes issue

Describe the change

It's possible for request.ContentLength to have a different value than the Content-Length header set on the request, and Go will only send the value defined on the request struct. This hardens the test to ensure the request struct is always correctly set.

How to verify it

Changelog text

• Fix: Make sure ContentLength is correctly set in the request.

Please verify and check that the pull request fulfills the following requirements

• Tests have been added or not applicable
• Documentation updates are included or not applicable (most documentation should be in the regclient.org repo)
• Changes have been rebased to main
• Multiple commits to the same code have been squashed
• All changes have been human generated or created with a reproducible tool