add nullable Bindings

[protogenes]

add overloads to JavaFxObserver and JavaFxSubscriber which allow to unmask null values in an Optional or guarded by a sentinel as counterpart to JavaFxObservable.nullableValuesOf

[protogenes]

I'm not sure if passing the exception to onError is the right approach

[rguillens]

It looks fine to me, BindingObserver.onError(throwable) apply user error management if provided or the exception is ignored.

[rguillens]

I don't see the point of this PR, maybe I don't understand very well what you want to achieve. I think the main idea behind JavaFxObservable.nullableValuesOf(final ObservableValue<T> fxObservable) and JavaFxObservable.valuesOf(final ObservableValue<T> fxObservable, final T nullSentinel) is to allow passing null values without emitting errors because RxJava 2 doesn't allows null emmissions. To emmit errors when null values occur you should use JavaFxObservable.valuesOf(ObservableValue<T> fxObservable). Please correct me if I'm wrong.

[protogenes]

Hi @rguillens,
the idea is to have the counterpart for nullableValuesOf. The JavaFxObserver methods convert a RX observable into a JavaFX binding, of which only the JavaFX bindings allow null values.
JavaFxObservable.nullableValuesOf has been there to hide those null values when building RX observables, but JavaFxObserver was missing methods which restore those null values when going back to a JavaFX binding.
This PR closes this gap.

[rguillens]

Now I understand your PR and the code looks very good. Congrats...

[thomasnield]

Hi @protogenes, thank you for contributing this. I'm speaking to KotlinConf this week so I may not be able to follow up until afterwards.

I will take a look at this and merge then.

[protogenes]

Hi @thomasnield how was KotlinConf?

[thomasnield]

@protogenes It was amazing, thank you. The talk will be uploaded to YouTube by JetBrains in the next week or two. The slides for my presentation are here: https://github.com/thomasnield/kotlinconf-datascience-talk

Regarding this PR, I'll try to take a look tonight alongside some PR's for RxKotlin.

[thomasnield]

I was just about to argue with this, but as I tried to write my alternative solution I get it now. I'll merge tomorrow after I review this with fresh eyes.

[thomasnield]

Good work, sorry it took me so long to get to this. It's been a busy year.

[thomasnield]

I'll do a release for RxJavaFX and RxKotlinFX.