[CRITICAL BUG] Fixed restoration bug caused by incorrectly check for unique identifier for vendor.

I have an live app that have user complaining that they couldn't access their purchase anymore.

From my diagnostic, it seems that after the initial purchase, if the user attempts to restore the subscription, it will always failed. After much debugging, I finally figure out the culprit. Below is a copy of the commit message I had attached to commit 37f83c7495515a729b63b6742af7483e5429b92d.

Every (re-)installation generates a new unique identifier for vendor.
Every purchase and restoration receipt will be tag with this new unique identifier.
But the latest receipt info might have a unique identifier for vendor from another device, from a previous installation, etc.
So we should only check if the purchase info matches receipt for device for receipt we restored with this device.
We should not check for unique identifier for vendor in latest receipt info.
This has caused a critical bug where restoration will always fails.

This commit attempts to fixed this issue.

Because I have already any pull request open, this commit got merge into PR #21. I'm sorry about that. I'm in the process on extracting out the PR to another branch and send another pull request.

Sorry about that!