encoding object with `/` field should drop other feilds

Here is an example code to reproduce the problem:

```js
const block = json.encode({
hello: {
'/': 'baguqeeratmwuhl736sndm4bi34xbifhyjqhatgwjrq6vjkfiafl7253rv4sq',
message: 'hello'
}
})

const data = json.decode(block) // Error: Invalid encoded CID form
```

I would expect encoder to just ignore `message` field of the `hello` and encode it as link as opposed to encoding it into an invalid block.