2.x: Check isdisposed before emitting in SingleFromCallable

[runningcode]

Previously SingleFromCallable did not check if the subscriber was
unsubscribed before emitting onSuccess or onError. This fixes that
behavior and adds tests to SingleFromCallable, CompletableFromCallable,
and MaybeFromCallable.

Fixes #5742

[akarnokd]

You got the error message wrong.

Travis CI

[runningcode]

👍 fixed now :)

[codecov]

Codecov Report

Merging #5743 into 2.x will decrease coverage by 0.06%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5743      +/-   ##
============================================
- Coverage     96.29%   96.22%   -0.07%     
- Complexity     5833     5836       +3     
============================================
  Files           634      634              
  Lines         41615    41620       +5     
  Branches       5761     5763       +2     
============================================
- Hits          40073    40050      -23     
- Misses          611      634      +23     
- Partials        931      936       +5

Impacted Files	Coverage Δ	Complexity Δ
.../internal/operators/single/SingleFromCallable.java	100% <100%> (ø)	5 <4> (+2)	⬆️
.../io/reactivex/internal/functions/ObjectHelper.java	94.73% <0%> (-5.27%)	20% <0%> (-1%)
.../operators/observable/ObservableFlatMapSingle.java	90.29% <0%> (-4.48%)	2% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	93.16% <0%> (-4.28%)	2% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	60.28% <0%> (-4.26%)	32% <0%> (-3%)
...ava/io/reactivex/processors/BehaviorProcessor.java	90.26% <0%> (-3.99%)	61% <0%> (ø)
...tivex/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...l/operators/observable/ObservableFlatMapMaybe.java	87.58% <0%> (-2.62%)	2% <0%> (ø)
...tivex/internal/schedulers/TrampolineScheduler.java	92.3% <0%> (-2.57%)	6% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	93.23% <0%> (-1.94%)	2% <0%> (ø)
... and 26 more

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[akarnokd]

Could you please restore the code coverage of SingleFromCallable? The new logic introduced a bunch of code paths that need testing.

[runningcode]

Sounds good. I added a few tests by copying the logic from ObservableFromCallable.