Rename ScheduledDisposable -> ScheduledSubscription

[bjorne]

Observable#subscribe_on incorrectly referred to the latter, but the class was actually called the former. Looking at similar implementation it seems to me to be more in line with a subscription than a disposable, so I updated the class description to match.

Added a simple test that uses #subscribe_on, and discovered that the test still failed when using the CurrentThreadScheduler, which seemed to be fixed by switching to curly brace syntax for the lambda in #unsubscribe.