Raising error in HTTP11Response if created with illegal attributes

[AvivC]

An HTTP response must either specify a content-length header, specify 'close' for a connection header to signal that the connection will be closed after the response, or be a chunked response.

If none of these conditions are true, we raise a detailed ValueError, instead of the plain assertion.

[Lukasa]

Thanks for this! The patch fails the linter: do you mind cleaning that up?