Ported groupByUntil function to scala-adapter

[chrisgrimm]

Ported the groupByUntil function to scala-adapter. This is my first time contributing, please let me know if anything is off.

[cloudbees-pull-request-builder]

RxJava-pull-requests #646 SUCCESS
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[samuelgruetter]

Thanks @chrisgrimm for contributing!

Some comments:

• If we add wildcards to the Duration Observable in Java (commit), then we can get rid of the cast in Scala (commit).
• There's an important difference between the signature we have in Java and your signature in Scala: In Java, the durationSelector gets a GroupedObservable, so it can extract the key if it wants to do so, but in Scala, it only gets an Observable, so it cannot know the key.
• When we add an operator to the Scala Observable, we usually also add an example to RxScalaDemo. These examples needn't test the functionality, because that's already done in Java, but they "test the signature": If you can write a meaningful example, then your signature is good. And these examples are also a useful piece of documentation.

[chrisgrimm]

@samuelgruetter
Okay, cool. I'll amend my changes to reflect these comments shortly.

[benjchristensen]

If you get these changes in today I'll release tomorrow (Tuesday US time).

[chrisgrimm]

@benjchristensen Sorry for the delay I just woke up. I added the example, changed the groupByUntil operator so that the conversion in the scala adapter wasn't necessary, and changed the signature of the closings function to include the key of the group in question.

[cloudbees-pull-request-builder]

RxJava-pull-requests #666 SUCCESS
This pull request looks good

[benjchristensen]

Thanks @chrisgrimm