Check if field is of same type as parent when expanding ModelAttribute

To avoid StackOverflowError when expanding ModelAttributes,
skip call to `expand` when a treeish field is encountered. #465
@digitil I think it should crash very obviously if the model attribute is recursively defined. There is no good way to generate a swagger model for a recursively defined type. I think the stack overflow seems like an appropriate behavior, unless someone can convince me otherwise :)