Kotlin boolean with 'is' prefix -> field name is wrong

If you declare a boolean property with an 'is' prefix like:
```kotlin
    var isPrimaryUser: Boolean? = false
```
... this is generated:
```ts
  primaryUser: boolean | null
```

Which is not correct and won't work, because the objects from the backend are serialized in an object like
```json
{ 
  isPrimaryUser: false 
}
```