2.x: Fix switchMap incorrect sync-fusion & error management

[akarnokd]

This PR fixes the incorrect request call inside switchMap when working with sync-fused sources.

Fixes: #6615

[vanniktech]

Déjà vu

[codecov]

Codecov Report

Merging #6618 into 2.x will increase coverage by 0.04%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##               2.x    #6618      +/-   ##
===========================================
+ Coverage     98.2%   98.25%   +0.04%     
- Complexity    6331     6337       +6     
===========================================
  Files          677      677              
  Lines        45506    45511       +5     
  Branches      6325     6326       +1     
===========================================
+ Hits         44691    44715      +24     
+ Misses         257      254       -3     
+ Partials       558      542      -16

Impacted Files	Coverage Δ	Complexity Δ
...internal/operators/flowable/FlowableSwitchMap.java	95.37% <100%> (-1.33%)	3 <0> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	91.53% <100%> (-3.15%)	3 <0> (ø)
.../operators/observable/ObservableFlatMapSingle.java	92.53% <0%> (-2.99%)	2% <0%> (ø)
...tivex/internal/schedulers/TrampolineScheduler.java	96.1% <0%> (-2.6%)	6% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	91.78% <0%> (-2.42%)	2% <0%> (ø)
...operators/observable/ObservableMergeWithMaybe.java	99.1% <0%> (-0.9%)	2% <0%> (ø)
...perators/single/SingleFlatMapIterableFlowable.java	96.66% <0%> (-0.84%)	2% <0%> (ø)
...vex/internal/operators/flowable/FlowableCache.java	97.72% <0%> (-0.76%)	37% <0%> (-1%)
... and 23 more

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