json-schema type alias translates to union in GraphQL

Hi, one additional issue... when you have a type definition in json-schema like this:
```
"DateTimeIso8601": {
"type": "string",
"description": "A datetime in ISO-8601 format (YYYY-MM-DDTHH:MM:SS.MMMZ)",
"format": "date-time"
}
```
It results in a union of String in graphql, like this: `union DateTimeIso8601 = String`
This is invalid graphql and results in an error:
```
Union type DateTimeIso8601 can only include Object types, it cannot include String.
```

Thanks