Added missing RVS Response Fields

[jordancco]

RVS Response examples: https://developer.amazon.com/docs/in-app-purchasing/iap-rvs-examples.html

[codecov]

Codecov Report

Merging #89 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #89   +/-   ##
=======================================
  Coverage   93.67%   93.67%           
=======================================
  Files           4        4           
  Lines         237      237           
=======================================
  Hits          222      222           
  Misses         10       10           
  Partials        5        5

Impacted Files	Coverage Δ
amazon/validator.go	94.11% <ø> (ø)	⬆️

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[jun06t]

Thanks @jordancco