Fix: display override when setting display and width props (xs, sm etc)

[aaronvanston]

• Checks to see if display is already set before adding display: block

Closes #77

[codecov]

Codecov Report

Merging #77 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #77   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          11     11           
  Lines          85     90    +5     
  Branches       21     23    +2     
=====================================
+ Hits           85     90    +5

Impacted Files	Coverage Δ
src/helpers/columnWidth/columnWidth.js	100% <100%> (ø)	⬆️
src/components/Col/Col.js	100% <100%> (ø)	⬆️

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