fix(NavItem): Maintain className passed to custom NavItems

[jeff-phillips-18]

What:
Keeps classNames set on the child of a custom NavItem.

Fixes #2133

[patternfly-build]

PatternFly-React preview: https://2134-pr-patternfly-react-patternfly.surge.sh

[codecov-io]

Codecov Report

Merging #2134 into master will increase coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2134      +/-   ##
==========================================
+ Coverage   80.96%   80.97%   +0.01%     
==========================================
  Files         649      649              
  Lines        7885     7885              
  Branches      505      505              
==========================================
+ Hits         6384     6385       +1     
  Misses       1261     1261              
+ Partials      240      239       -1

Flag	Coverage Δ
#patternfly3	85.22% <ø> (ø)	⬆️
#patternfly4	76.3% <0%> (+0.02%)	⬆️
#patternflymisc	95.68% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
...ernfly-4/react-core/src/components/Nav/NavItem.tsx	57.14% <0%> (+3.57%)	⬆️

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[redallen]

Awesome, thanks!

[rebeccaalpert]

Looks great to me.

[tlabaj]

LGTM