…ame`.

  u = URI("http://[::1]/bar")
  p u.hostname      #=> "::1"
  p u.host          #=> "[::1]"

With IPv6, any time we have a string which represents an authority, as
defined by RFC7540, the address must be contained within square brackets,
e.g.: "[2020::1985]:443". Representations from the `host` header and
`authority` pseudo-header must conform to this format.

Some headers, notably `x-forwarded-for` and `x-forwarded-host` do not
format the authority correctly. So we introduce a private method
`wrap_ipv6` which uses a heuristic to detect these situations and fix the
formatting.

Additionally, we introduce some new assertions in `Rack::Lint` to ensure
SREVER_NAME and HTTP_HOST match the formatting requirements.