support deep unions in lists

[Danny02]

I noticed that you merge the schemas of maps in lists.

So that for example the document

{
  "list": [
    {"a": "some value"},
    {"b": "other value"},
  ]
}

creates an inner schema where a and b are optional.

I noticed with some of my data that this is not done recursivly, for example this document does not work currently

{
  "list": [
    {"inner": {"a": "some value"}},
    {"inner": {"b": "other value"}},
  ]
}

This PR "fixes" this.