Make PaymentState.state and transaction public.

[yanyin1986]

Hi, with InAppPurchase v2.1.4.
I did validate the transaction which returned by the callback of addTransactionObserver.
After v2.3.0, I found I can't do that anymore.

It will be work after making state and transaction public.
Thanks.

[codecov]

Codecov Report

Merging #44 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #44   +/-   ##
=======================================
  Coverage   95.25%   95.25%           
=======================================
  Files           6        6           
  Lines         253      253           
=======================================
  Hits          241      241           
  Misses         12       12

Impacted Files	Coverage Δ
Sources/InAppPurchase.swift	100% <ø> (ø)	⬆️

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[calda]

+1, cd4f179 broke this important functionality.

[mdcdeveloper]

+1, I have to send to server side the transactionIdentifier, that's the identifier in our app. Waiting to merge! Please! Anyway when I print it it print's out the id's so I don't see why this is not public. Thanks in advance!

[glizik]

This is essential.

[jinSasaki]

Thank you for the pull request. This is my mistake.
Merge and release a new version.