fix(core): allow extra spaces in headerCssClass

[ghiscoding]

Follows SlickGrid PR 960

Extra spaces in class dom attribute are allowed in html (e.g. class="some-class other-class "). SlickGrid should probably also behave the same way (or at least fail with some meaningfull validation error).

This commit just ignores all extra spaces in headerCssClass (between individual classes or leading / trailing).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b0102b5) 94.7% compared to head (77ac541) 94.7%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1303     +/-   ##
========================================
+ Coverage    94.7%   94.7%   +0.1%     
========================================
  Files         198     198             
  Lines       21314   21317      +3     
  Branches     7087    7085      -2     
========================================
+ Hits        20182   20185      +3     
  Misses       1132    1132             

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