ipv6 localhost address is passed to hackney without square brackets

I am trying to build a simple reverse proxy with https://hexdocs.pm/reverse_proxy_plug and its httpoison plugin. I am trying to forward requests to a (angular dev-). server, listening on localhost:4200.
As localhost is ipv6 on macOS and hackney does not seem to resolve this correctly (see https://github.com/benoitc/hackney/issues/718), I set the upstream to "http://[::1]:4200/" which reaches :hackney.request/5 as "http://::1:4200" and causes
```
1st argument: not a textual representation of an integer

:erlang.list_to_integer(':1:4200')
(hackney 1.18.1) /Users/philipp/wspc/elixir/rest_api/deps/hackney/src/hackney_url.erl:260: :hackney_url.parse_netloc/2
(hackney 1.18.1) /Users/philipp/wspc/elixir/rest_api/deps/hackney/src/hackney.erl:351: :hackney.request/5
```
IMHO the square brackets must not be removed from the url.

used versions:
OTP 25
Elixir 1.14.1
Httpoison 2.1.0
hackney 1.18.1

Reproduce the failure with the upstream URL "http://[::1]:4200/" and inspect the stack trace around hackney_url.erl:260 and hackney.erl:351. Trace how Httpoison passes the URL to hackney; done means the IPv6 address retains its square brackets and the request reaches the localhost server.

Written by the indexing model from the issue text.