Prevent auto_register in Server from being wrongly set to False

[ghost]

Resolves #441

The attribute auto_register in Server and its inherited classes is used to register/unregister the Server to a registry.
If set to True, it will :
1. set itself to False in the _register() method
2. spawn _bg_register() in the _register() method
3. call registrar.unregister() in the close() method

Actions 1 and 3 are conflicting because 1 will prevent 3 from ever happening.
By removing 1, the auto_register mechanism will work correctly, without any side-effects because it is not called anywhere else.

Test case : I haven't had time to add one yet, but if necessary I will look into it.

[comrumino]

The commit that introduced the conflicting steps was 3a3d0ec5. I can't see any obvious reason the changes made so far would be problematic.

[comrumino]

Since there isn't existing testing for auto_register, I'm going to go ahead and merge this. Thanks!