Maps are not copied

[nschoe]

When extending an object into a second, the Map values of the first don't get copied.

[unclechu]

@nschoe I'm sorry but I won't implement this in current version of this package, because it's really out of JSONy pretty basic types. But I'm about to develop new version that supposed to support custom middlewares for specific types, but I can't tell you when it's going to be done. I'm sorry for so big delay for the answer, I'm not very enthusiastic about javascript these days, if you really need this behavior you could fork this package for yourself for example.