Add ConvertableKeep()

[kazu]

ConvertableKeep is not checking on keep.

[codecov-commenter]

Codecov Report

Merging #11 (2ab15d6) into master (ecd95e6) will increase coverage by 0.17%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #11      +/-   ##
==========================================
+ Coverage   81.81%   81.99%   +0.17%     
==========================================
  Files          12       12              
  Lines         407      411       +4     
==========================================
+ Hits          333      337       +4     
  Misses         43       43              
  Partials       31       31              

Impacted Files	Coverage Δ
currying.go	92.98% <100.00%> (+0.52%)	⬆️

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