Support for IPv6 addresses

[gh0st42]

While switching my code from IPv4 only to mixed mode as well as IPv6 only mode, I encountered a strange bug in attohttpc.

At first, I thought it would be as easy as replacing strings such as "http://127.0.0.1:3000/fancy_rest" with "http://[::1]:3000/fancy_rest". The latter works in my browser and in curl. Even the url crate can parse it correctly.

Just when I use attohttpc I get the following error on Linux:

Error(Io(Custom { kind: Other, error: "failed to lookup address information: Name or service not known" }))

on other systems it might also be displayed like the following:

Error(Io(Custom { kind: Other, error: "failed to lookup address information: Temporary failure in name resolution" })) 

Probably trying to interpret [::1] as a hostname instead of an IpAddr.

Unfortunately, attohttpc is not yet available on the rust playground, otherwise, I would have shown a minimal example there.

And totally unrelated: thank you for providing a pretty neat, slim and efficient http client library without async+await :D

[adamreichold]

I think the problem is that Ipv6Addr::from_str does not parse the [...] notation, but we use Url::host_str which hands out this notation instead of using Url::host which gives either an already parsed address or a domain name. Will prepare an MR to switch us to Url::host...