Fix ipv6 parser

[m-sp]

should hopefully fix #4145 (review)

but the following test fails:

    "only parse part of an invalid ipv6 address where a single section is shortened (must be 2 or more)" in {
      val invalidIp = "2001:db8::1:1:1:1:1"
      Ipv6Address.fromString(invalidIp).map(_.value) must beLeft
    }

but I don't really see why the ip address is invalid

[rossabaker]

The failing test does parse in non-http4s decoders.

[rossabaker]

Why (0, 3) instead of (3, 3)?

[m-sp]

it would fail on e.g. 53f6::1:551b:7bd8

the number of segments after :: are always fixed to a number n. so before the :: we can parse up to 8 - n segments

[rossabaker]

Ah, right. So this is part of the bug fix.

[rossabaker]

Oh, that address was invalid because a :: is supposed to represent multiple groups of zeroes. Strictly, the test was correct, but I don't think it ambiguates anything to accept it.

[m-sp]

Oh, that address was invalid because a :: is supposed to represent multiple groups of zeroes. Strictly, the test was correct, but I don't think it ambiguates anything to accept it.

I have an alternative fix here #4243 which also handles that case and IMO is a bit easier to understand. let me know which one you prefer and I can resolve the conflicts afterwards