feat(Dropdown): add allowReselection prop

[ilyaliao]

Fixes https://github.com/Semantic-Org/Semantic-UI-React/issues/4415

This PR adds the allowReselection prop to the Dropdown. This enables user to trigger onChange events even when reselecting same value.

I think I've resolved the issue. If there are any omissions or areas that need modification, feel free to provide suggestions!

[welcome[bot]]

💖 Thanks for opening this pull request! 💖

Here is a list of things that will help get it across the finish line:

• Run yarn lint locally to catch formatting errors. This will fix some errors automatically, commit and push any changes.
• Run yarn test locally to catch errors. This ensures all components still behave as they should.
• Run yarn start to run the doc site locally and try a few pages, ensuring everything is in good working order.
• Include tests when adding/changing behavior.

We get a lot of pull requests on this repo, so please be patient and we will get back to you as soon as we can.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (e243fa4) 99.51% compared to head (1c170d8) 99.51%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4450      +/-   ##
==========================================
- Coverage   99.51%   99.51%   -0.01%     
==========================================
  Files         186      186              
  Lines        3512     3511       -1     
==========================================
- Hits         3495     3494       -1     
  Misses         17       17              

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