Single enum variable validation message

[thebitrock]

fieldName: { type: String, enum: ['A'] },
return message 'fieldName must be either or A.'

[codecov]

Codecov Report

Merging #115 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #115   +/-   ##
=======================================
  Coverage   98.79%   98.79%           
=======================================
  Files           6        6           
  Lines         248      248           
=======================================
  Hits          245      245           
  Misses          3        3           

Impacted Files	Coverage Δ
src/messages.js	89.65% <100.00%> (ø)

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