Fix: Socket.unix

[maiha]

Socket.unix is broken due to a tiny typo, and it has no references from spec.

in src/socket.cr:55: undefined constant Type::Stream (did you mean 'Type::STREAM')
  def self.unix(type : Type = Type::Stream, blocking = false)

This PR

• Fixed Socket.unix
• Added a simple spec for it to avoid dead code

Socket.unix.family # => UNIX

Thanks.

[ysbaddaden]

Thank you!