ReactiveX / rxjs / 6675

97%

master: 97%

Pull #2479

travis-ci

fix(mergeAll): add source subscription to composite before actually subscribing

Add subscriptions for source Observables to mergeAll composite subscription
before actually subscribing to any of these Observables, so that if
source Observable emits synchronously and consumer of mergeAll unsubscribes
at that moment (for example `take` operator), subscription to source is
unsubscribed as well and Observable stops emitting.

Closes #2476

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