Fix: Handle IPv6 hostname correctly (closes #806)

[JohannaHartmann]

Wrapped the hostname in brackets to separate it from the port, when it is an IPv6 address.
It is an attempt to fix the issue #806.

Is the net_http_spec.rb the right place for the added specs or should I rather put them into a new file net_http_utility_spec.rb?

[bblimke]

Thank you @JohannaHartmann

In terms of specs, I think ideally it would be to add generic specs for stubbing and request expectations
and enable them for each client that supports IPv6, but this will likely require changes in all adapters.
For now I would at least add a few examples like you added to net_http_spec.rb, but specs that work against WebMock API,
e.g. using stub_request or expect request and don't know about WebMock internals like NetHttpUtility

[bblimke]

Thank you @JohannaHartmann !

[bblimke]

It's released as 3.12.1