🐛 Fixed creation of extra customer when updating plans
🐛 Fixed creation of extra customer when updating plans

no issue

- `customers` property contains an array of customer for which 'for..of' syntax is more appropriate
- Bug was causing creation of multiple customers in Stripe when new checkout session was initiated for existing customer
- Discussed in https://github.com/TryGhost/Members/pull/90/files#r368889289

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