Add a test for #280

[dralley]

Currently fails: #280

[Mingun]

As I already said in Mingun/fast-xml#2, it is better to test actual result and not roundtrips, because deserializer is able to deserialize different shaped into the same structure, and for serializer we usually want some exact format (because usually XML has a scheme and that scheme is not under our control).

Because I'll plan to look serializer issues more closely when I'll finish with deserializer, I suggest close this for now.

[dralley]

I'll close it for now, once I get some free time on a weekend I will try to address your feedback, presuming you don't get to it first :)