fix(Dropdown): prevent calling onChange unless value changed

[zarend]

For the Dropdown component, ensures that onChange is called only if the user selectes a value that is different from the current value of the Dropdown. This is so we can have parity with the regular HTML select.

[welcome]

💖 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-io]

Codecov Report

Merging #3391 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3391      +/-   ##
==========================================
+ Coverage   99.89%   99.89%   +<.01%     
==========================================
  Files         170      170              
  Lines        2809     2812       +3     
==========================================
+ Hits         2806     2809       +3     
  Misses          3        3

Impacted Files	Coverage Δ
src/modules/Dropdown/Dropdown.js	99.76% <100%> (ø)	⬆️
src/modules/Popup/Popup.js	99.23% <0%> (ø)	⬆️

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[spencer-brown]

awesome. thanks, @zarend !

[welcome]

Congrats on merging your first pull request! 🎉🎉🎉

[levithomason]

Excellent work, thank you much!

[levithomason]

Released in [email protected].