Add Missing NewNumberSetAttribute

[daviskoh]

Changes:

• add NewNumberSetAttribute
• add test

Fixes #157

[codecov-io]

Codecov Report

Merging #167 into master will increase coverage by 0.22%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #167      +/-   ##
==========================================
+ Coverage    69.8%   70.02%   +0.22%     
==========================================
  Files          18       18              
  Lines         679      684       +5     
==========================================
+ Hits          474      479       +5     
  Misses        165      165              
  Partials       40       40

Impacted Files	Coverage Δ
events/attributevalue.go	82.8% <100%> (+0.3%)	⬆️

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[bmoffatt]

LGTM