Made OpenApiVisitorBase Enter() and Exit() virtual, plus test

[peteraritchie]

I was working on some processing ideas and using the decorator pattern to chain together different algorithms. I realized that a decorator can't pass through to all the methods the walker would call. So, a decorated class doesn't get all the information that an undecoreated class (i.e. Enter and Exit, PathString and CurrentKeys.

This pull request makes Enter and Exit virtual and includes a test to make sure the extra two virtual methods an involved.

[darrelmiller]

I like this. I still somewhat regret breaking the walker and visitor into two classes, but this helps.

[peteraritchie]

I like this. I still somewhat regret breaking the walker and visitor into two classes, but this helps.

Would it still be a visitor if the object structure wasn't separated from the algorithm? ;)