allOf is not properly merged

Given a schema that uses `allOf` to combine two different `object` schemas, the properties of the two schemas are not combined:

```json
{
"components": {
"schemas": {
"identifier": {
"type": "object",
"properties": {
"id": {"type": "string"}
}
},
"person": {
"allOf": [
{"$ref": "#/components/schemas/identifier"},
{
"type": "object",
"properties": {
"name": {
"type": "string"
}
}
}
]
}
}
}
}
```

Not 100% sure that schema validates, but it should be a good start.