Kong plugin schemas support different type of fields,
including maps. Moreover, these can either be arbitrary
maps, or maps with their own subschema.

go-kong already supports well the second case, but it
falls short with arbitrary maps. An example of arbitrary
map field is the `custom_fields_by_lua` from the `http-log`
plugin, which is defined as follows:

```
{
  "custom_fields_by_lua": {
    "keys": {
      "len_min": 1,
      "type": "string"
    },
    "type": "map",
    "values": {
      "len_min": 1,
      "type": "string"
    }
  }
}
```

This commit makes sure that go-kong is able to correctly
handle such schemas as well.