JSON-LD parser does not like relative URIs

Relative URIs are not resolved correctly.

Parsing /tmp/file.jsonld:

```
{
  "@context": {
    "@vocab": "http://schema.org/",
    "url":
    {
        "@type": "@id"
    }
  },
  "url": "../organization/ibm"
}
```

leads to:

```
_:bn <http://schema.org/url> <http://schema.org/file/tmp/organization/ibm>
```
