Fix HTTP test outcome

[scintillavoy]

Hi there! Thanks for providing great Rust workshops, including this one.

I found a small issue in the test outcome and fixed it. In the test errors_on_invalid_content_length:

• Content-Length should not match the length of the body.
• Content-Type should be included, since it's not part of the test criteria.

This change should also be applied to the solutions branch.