fix(extension): registerPlugin not implemented correctly

[ghiscoding]

No description provided.

[codecov]

Codecov Report

Merging #482 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            master      #482    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files          148       148            
  Lines         9542      9545     +3     
  Branches      3321      3187   -134     
==========================================
+ Hits          9542      9545     +3     

Impacted Files	Coverage Δ
...es/angular-slickgrid/services/extension.service.ts	100.00% <100.00%> (ø)

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