fix(export): add grouped header title (from pre-header) into exports

[ghiscoding]

• when having Grouped Header Titles (in pre-header), those were not showing in the export, now they do

[codecov]

Codecov Report

Merging #436 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #436   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          141       141           
  Lines         8899      8956   +57     
  Branches      3075      3099   +24     
=========================================
+ Hits          8899      8956   +57     

Impacted Files	Coverage Δ
...p/modules/angular-slickgrid/global-grid-options.ts	100.00% <ø> (ø)
.../angular-slickgrid/services/excelExport.service.ts	100.00% <100.00%> (ø)
...dules/angular-slickgrid/services/export.service.ts	100.00% <100.00%> (ø)

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