Allow for debug verification of content-length to be disabled

[zachmatson]

To test the ability of some code to handle responses with invalid content-length, I want to provide a false value for this header and serve that in my test code. Unfortunately, hyper has this debug assertion which fails when trying to serve a request where the content-length header has been manually overridden to an incorrect value.

It would be nice if there was some way to disable this check, or a workaround for this use case.