Add missing fields in IAPResponseForIOS6

[OmarElgabry]

Fixes: #71

[codecov]

Codecov Report

Merging #121 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #121   +/-   ##
=======================================
  Coverage   87.10%   87.10%           
=======================================
  Files           4        4           
  Lines         256      256           
=======================================
  Hits          223      223           
  Misses         22       22           
  Partials       11       11           

Impacted Files	Coverage Δ
appstore/model.go	100.00% <ø> (ø)

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[jun06t]

@OmarElgabry
thanks a lot :-)