2.x: Zip, CombineLatest, and Amb operators throw when supplied with ObservableSource implementation that doesn't subclass Observable

[mgsholte]

Fixes #6753

[akarnokd]

Please add unit tests so this doesn't happen again in case of any future change.

[codecov]

Codecov Report

Merging #6754 into 2.x will decrease coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #6754      +/-   ##
============================================
- Coverage     98.25%   98.24%   -0.01%     
+ Complexity     6350     6347       -3     
============================================
  Files           677      677              
  Lines         45532    45532              
  Branches       6334     6334              
============================================
- Hits          44736    44732       -4     
- Misses          247      248       +1     
- Partials        549      552       +3

Impacted Files	Coverage Δ	Complexity Δ
...x/internal/operators/observable/ObservableZip.java	100% <100%> (ø)	6 <0> (ø)	⬇️
.../operators/observable/ObservableCombineLatest.java	96.31% <100%> (-0.62%)	6 <0> (ø)
...x/internal/operators/observable/ObservableAmb.java	98.94% <100%> (ø)	8 <0> (ø)	⬇️
...ernal/operators/flowable/FlowableFlatMapMaybe.java	90.82% <0%> (-5.32%)	2% <0%> (ø)
...tivex/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...nal/operators/observable/ObservablePublishAlt.java	92.52% <0%> (-3.74%)	14% <0%> (-1%)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...l/operators/observable/ObservableFlatMapMaybe.java	88.23% <0%> (-1.97%)	2% <0%> (ø)
.../io/reactivex/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
...ternal/operators/flowable/FlowableSubscribeOn.java	96.61% <0%> (-1.7%)	2% <0%> (ø)
... and 18 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 31b407f...b48d0f5. Read the comment docs.

[mgsholte]

Added unit tests.