#20 Add a MustSet function

[BoynChan]

#20 Add a MustSet function.
I add a MustSet function in defaults.go and did some tests in its test file.

[codecov-commenter]

Codecov Report

Merging #22 into master will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #22      +/-   ##
==========================================
+ Coverage   98.26%   98.30%   +0.04%     
==========================================
  Files           2        2              
  Lines         115      118       +3     
==========================================
+ Hits          113      116       +3     
  Misses          1        1              
  Partials        1        1              

Impacted Files	Coverage Δ
defaults.go	98.26% <100.00%> (+0.04%)	⬆️

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[creasty]

Thank you 👍