Add missing className to wrapper

[gpbl]

Closes #432

[codecov]

Codecov Report

Merging #434 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #434   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          15     15           
  Lines         490    490           
  Branches       99     99           
=====================================
  Hits          490    490

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

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