nullable does not get converted correctly

in a swagger 2.0 yaml file:
```yaml
# some property in a definition
datumStart:
type: string
format: date
example: '2019-01-13'
nullable: true
```

when converted to schema:

## Actual output

```json
"datumStart": {
"nullable": true,
"type": "string",
"example": "2019-01-13",
"format": "date"
},
```

## Expected output

```json
"datumStart": {
"type": ["string", "null"],
"example": "2019-01-13",
"format": "date"
},
```