1.x: fix broken backpressure through unsubscribeOn()

[akarnokd]

The unsubscribeOn did not properly forward the upstream's Producer to the downstream, thus the default Subscriber request behavior of Long.MAX_VALUE was incorrectly in effect.

Fixes: #5725

[codecov]

Codecov Report

Merging #5726 into 1.x will increase coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                1.x    #5726      +/-   ##
============================================
+ Coverage     84.09%   84.12%   +0.02%     
+ Complexity     2886     2883       -3     
============================================
  Files           290      290              
  Lines         18256    18258       +2     
  Branches       2495     2495              
============================================
+ Hits          15353    15360       +7     
+ Misses         2017     2008       -9     
- Partials        886      890       +4

Impacted Files	Coverage Δ	Complexity Δ
...a/rx/internal/operators/OperatorUnsubscribeOn.java	90% <100%> (+1.11%)	2 <0> (ø)	⬇️
...n/java/rx/subscriptions/CompositeSubscription.java	74.02% <0%> (-3.9%)	24% <0%> (-2%)
.../java/rx/internal/operators/BackpressureUtils.java	68.18% <0%> (-2.28%)	27% <0%> (-1%)
.../main/java/rx/internal/util/IndexedRingBuffer.java	61.53% <0%> (-1.54%)	26% <0%> (ø)
...n/java/rx/subjects/SubjectSubscriptionManager.java	80.71% <0%> (-1.43%)	15% <0%> (-1%)
...n/java/rx/internal/operators/CachedObservable.java	81.15% <0%> (-0.53%)	6% <0%> (ø)
...x/internal/operators/OnSubscribeCombineLatest.java	77.88% <0%> (-0.51%)	4% <0%> (ø)
...main/java/rx/internal/operators/OperatorMerge.java	85.32% <0%> (-0.46%)	7% <0%> (ø)
.../rx/internal/schedulers/CachedThreadScheduler.java	91.26% <0%> (+0.97%)	6% <0%> (ø)	⬇️
src/main/java/rx/observers/SerializedObserver.java	97.82% <0%> (+1.08%)	19% <0%> (+1%)	⬆️
... and 4 more

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