fix(filters): Grid State filters should always include an operator

[ghiscoding]

No description provided.

[codecov]

Codecov Report

Merging #676 (c4a70e7) into master (e3e623d) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #676   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          157       157           
  Lines        10434     10435    +1     
  Branches      3661      3665    +4     
=========================================
+ Hits         10434     10435    +1     

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

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