Added socket.disconnect()

[britzl]

Added function to disconnect a previously connected socket:

-- create socket
local socket = client.create_socket()

-- connect socket
local ok, err = socket.connect()
if not ok then
    return
end

-- disconnect socket
socket.disconnect()

Fix #84

[DannyIsYog]

This assert will always pass, is this intended? Isn't there a better way to assert if the sockets was indeed disconnected?

Like:

assert_nil(socket.connection)

[britzl]

Good point. Let me take a look.

[britzl]

@DannyIsYog I've updated the test with a better assert. Could you please take another look?

[DannyIsYog]

Looks good!

[britzl]

Thanks. @DannyIsYog what does is the correct release process for this repository when there are external collaborators such as myself? I am able to create a new release, but I also need to update the changelog to assign the unreleased changes to a release prior to actually making the release itself. Do I first create a separate PR with the changelog update and then do the release?