Fix the bug that Switch doesn't propagate 'unsubscribe'
OperatorScan should check for MAX_VALUE on request
Ignore furthur messages after entering terminate state
Error Handling Unsubscribe and Terminal State
Make TakeUntil obey Rx contract
0.20.4 and 1.0.0-rc.3
Merge pull request #1672 from benjchristensen/merge-0_20_4

Merge 0.20.4 Fixes