Make stringify protected about assert

[mdavis1982]

Why do you need to subclass the Assertion class? I can't think of a valid use case for doing this.

[sagikazarmark]

This contradicts with the readme:

https://github.com/beberlei/assert#your-own-assertion-class

The obvious answer: custom assertions.

[mdavis1982]

You can still use custom assertions without needing to access the stringify() method. I can't see a use case where you need to subclass the Assertion class and then access the stringify() method. What is the use case for this?

[sagikazarmark]

Since this issue is about 6 months old, I don't recall the exact use case. However, I can imagine custom, complex assertion which uses that method. Since the whole Assertion class uses it, I don't think it's hard to find one custom use case. Any case, you want to do something with strings.

[mdavis1982]

I think we need to refer this back to @beberlei - but I'm not sure if changing it would have any other implications.

[beberlei]

this was done in some PR.