Add option to return redirect URL as a header in verifyRequest

[paulomarg]

WHY are these changes introduced?

Fixes Shopify/shopify-app-template-node#575

Currently, the verifyRequest middleware will always return the code to redirect the user back to /auth, even if the request is not for a page. In those cases, we still want to fail the request, but we need the frontend to be able to pick up on this and trigger the OAuth flow.

WHAT is this pull request doing?

This adds a returnHeader setting to verifyRequest, which will fail the request but return the necessary data for a redirect as a header, rather than the actual redirection code. That will allow the middleware to be used for both page loads and XHR requests.

Type of change

• Patch: Bug (non-breaking change which fixes an issue)
• Minor: New feature (non-breaking change which adds functionality)
• Major: Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have added a changelog entry, prefixed by the type of change noted above
• I have added/updated tests for this change

[mllemango]

Will there be a readme update?