Make sure that the order gateway is check if the level is check only

[dparker1005]

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Changes proposed in this Pull Request:

Resolves #132

How to test the changes in this Pull Request:

Steps to reproduce the behavior:

Have Stripe (Checkout), the PayPal Express Add On and the Pay by Check Add On active
My level had an initial amount, expiration date, and the 'Allow Paying by Check' setting set to 'Users can only pay by check'
Checkout with this level
You'll automatically be redirected to Stripe Checkout

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.