Allow empty paths to merge nested data into the top level.

[seyfahni]

Deserialization of properties marked with #[Json(path: []) did already work, but serialization did not.

I wrote tests to verify this behavior and adjusted the Json class to make serialization work too.

My tests cover the following cases (both serialization and deserialization):

• object contains a property that has an empty path and is merged into the containing objects json data
• multiple levels of zero-path merging
• multiple objects that have the same path prefix are merged together

You can of course decide if you want this to be officially possible.

[CLAassistant]

All committers have signed the CLA.

[khepin]

I'll have to sit on this one for a while. I'm not fully convinced this is something we'd want here

[seyfahni]

Yeah, I expected that. But it allows for some quite nice code sharing.

I created this for serializing a request that looks like one big json object, but has a few fields that are decided based on another values content. And with this you can just inline those values instead or duplicating much of the code (all the other values).