Allow comparison and serialization with sub-classes.

[Jack12816]

First things first: Thank you very much for this great gem!

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- What is it good for

With the small changes, we can not inherit the Tree::TreeNode class and build specialized trees (with extended functionality) and serialize and compare them correctly. Before the deserialization always forced the children to be of type Tree::TreeNode, which drops custom functionality.

- What I did

• Added support for correct sub-class (de)serialization
• Improved the comparability, so we allow pure Tree::TreeNode instances to be compared with compatible sub-classes
• Added specs for both things

- A picture of a cute animal (not mandatory but encouraged)

[evolve75]

Thanks @Jack12816. This is very helpful. I have merged your changes and it
should appear in RubyTree R2.0.0.