withLatestFrom() build error with RxKotlin 2.4.0

I just updated RxKotlin from 2.3.0 to 2.4.0 and this code no longer compiles.

```kotlin
Observable.just(Unit)
    .withLatestFrom(this) { _, item: T -> item }
```
Now I have to write
```kotlin
Observable.just(Unit)
    .withLatestFrom(this, BiFunction<Unit, T, T> { _, item -> item })
```

Kotlin 1.3.41.

Possibly related to #196?