Hi all!

I was testing some static analysis tool and decided to use the HTTP module as testing ground. While running `bandit` at the client module, it detected 3 instances of using `assert` inside the code. Twice in the HTTPResponse class and once in the HTTPConnection class.

Now, I know that this will only cause any trouble when running python with the optimize settings turned on and if someone is that concerned about optimization, they probably won't be using the stdlib's HTTP implementation, but I think it would be fitting to fix this corner case.

I've written a PR that fixes this but I'm not sure if the raised exceptions and messages are ok