Link container requires rel type

The following code

```
WoT.produce({
...
links: [ {
href: "https://www.thingweb.io/img/favicon/favicon.png",
sizes: "16x14",
// rel: "icon", // required to succeed
} ],
...
}
```

fails with

```
Error: Cannot produce ExposedThing because " + Thing Description JSON schema validation failed:
should NOT be valid
should have required property 'rel'
should match exactly one schema in oneOf
```

This should **NOT** be the case since `rel` is optional.
see https://w3c.github.io/wot-thing-description/#link