Subscription renewal doubles line item tax

[pogla]

Steps to reproduce

1. Update the plugins to the latest versions. It also doesn't work with older versions.

2. Create a subscription. Use a delivery address that pulls tax rate via the TaxJar API. We are using a tax class on the product but I don't think it matters.
3. Process renewal

4. Line item gets double taxes applied.

5. If you press Recalculate, it gets recalculated properly.

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I traced down the issue to this loop. So the loop will add a tax line item for each existing line item. Even if the tax item already exists.

[dallendalton]

This is resolved with the fix in #179