Fix IPv6 hostname normalization

[lidel]

The old check effectively merged the meaning of host and hostname, which was prone to errors:

• in rare scenarios host could include : but be ipv4 with a custom port (eg. 127.0.0.1:8080) which would produce false-positives
• ipv6 could already have square brackets([::1] – see the end of this comment), but normalization would run anyway and produced invalid URIs like http://[[::1]]:80
  • this caused issues like example in the wild

This changes ipv6 normalization to only run if actual hostname parameter is provided, includes : and does not start with [. Also, added missing tests for ipv6.

cc #18, https://github.com/ipfs-shipyard/ipfs-companion/issues/668