add border width for box component

[peammm]

I need to use border in box component but It missing border width, so I add border width for box component and update test case

[kkdai]

LGTM, thank you.

[codecov-commenter]

Codecov Report

Merging #294 (6d9ff0d) into master (f8b2c19) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #294      +/-   ##
==========================================
+ Coverage   78.48%   78.50%   +0.01%     
==========================================
  Files          37       37              
  Lines        2868     2870       +2     
==========================================
+ Hits         2251     2253       +2     
  Misses        486      486              
  Partials      131      131              

Impacted Files	Coverage Δ
linebot/flex.go	59.23% <100.00%> (+0.28%)	⬆️

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