unsubscribe before retry
Unsubscribe on retry using SerialSubscription
Merge pull request #1019 from benjchristensen/retry-1018

Fix: retry() never unsubscribes from source until operator completes
[Gradle Release Plugin] - pre tag commit:  '0.17.5'.
[Gradle Release Plugin] - new version commit:  '0.17.6-SNAPSHOT'.
Version 0.17.5