Force LoadingNotificationCommand to execute the then argument on ajax completion

[jvanecek]

Proposed solution for #207

[codecov]

Codecov Report

Merging #208 (b0b0ef4) into release-candidate (ce05e1b) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@                  Coverage Diff                  @@
##           release-candidate     #208      +/-   ##
=====================================================
- Coverage              99.96%   99.96%   -0.01%     
=====================================================
  Files                    229      229              
  Lines                  22078    22052      -26     
=====================================================
- Hits                   22070    22044      -26     
  Misses                     8        8              

Impacted Files	Coverage Δ
...sts/CombinedWebInteractionInterpreterTest.class.st	100.00% <100.00%> (ø)
...Core-Tests/LoadingNotificationCommandTest.class.st	100.00% <100.00%> (ø)
...-Core-Tests/WebInteractionInterpreterTest.class.st	100.00% <100.00%> (ø)
...ce/Willow-Core/LoadingNotificationCommand.class.st	100.00% <100.00%> (ø)

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