Taxes attribute is bad formed in JSON response

When recovering an invoice via API, "taxes" attribute is bad formed:

```
"taxes": {
   "{\\"id\\"=>4, \\"name\\"=>\\"VAT 21%\\", \\"value\\"=>\\"21.0\\", \\"active\\"=>true, \\"default\\"=>true, \\"deleted_at\\"=>nil}":"1.89"
}
```

Expected value:
```
"taxes": [{
    "id": 4,
    "name": "VAT 21%",
    "value": 21.0,
    "active": true,
    "default": true
}]
```
