fix(Button): fix handling of icon class

[layershifter]

Fixes #1691.

[codecov-io]

Codecov Report

Merging #1693 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1693      +/-   ##
==========================================
+ Coverage   99.75%   99.75%   +<.01%     
==========================================
  Files         141      141              
  Lines        2412     2414       +2     
==========================================
+ Hits         2406     2408       +2     
  Misses          6        6

Impacted Files	Coverage Δ
src/elements/Button/Button.js	100% <100%> (ø)	⬆️

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[levithomason]

Released in [email protected].