IPv6 upstream members produce invalid configuration

[silkeh]

Affected Puppet, Ruby, OS and module versions/distributions

• Puppet: 4.8.3
• Ruby: 2.3.3p222
• Distribution: Debian
• Module version: 0.15.0

How to reproduce (e.g Puppet code you use)

Define an upstream with an IPv6 address:

nginx::resource::upstream { 'example':
  members => {
    'ip6-localhost:8008' => {
      server => '::1',
      port   => 1234,
    }
  },
}

What are you seeing

Produced configuration is invalid, specifically:

 # MANAGED BY PUPPET
 upstream example {
  server ::1:1234;
 }

What behaviour did you expect instead

Valid configuration, specifically:

 # MANAGED BY PUPPET
 upstream example {
  server [::1]:1234;
 }