Set Access-Control-Allow-Credentials for serverless mode

[elliottsj]

This enables Ackee v3 to work on Vercel. Should work for Netlify too, though I've only tested Vercel.

Fixes #223

See https://github.com/apollographql/apollo-server/blob/d63afe1b03d8bd0d4c8a4c572c8f7bed23b6d270/packages/apollo-server-lambda/src/ApolloServer.ts#L106-L108

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[coveralls]

Pull Request Test Coverage Report for Build 546056388

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-2.0%) to 85.222%

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Totals
Change from base Build 507221259:	-2.0%
Covered Lines:	1186
Relevant Lines:	1300

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💛 - Coveralls

[electerious]

Thanks for the PR!

It seems that apollo sets the credentials header when using this option (https://github.com/apollographql/apollo-server/blob/d63afe1b03d8bd0d4c8a4c572c8f7bed23b6d270/packages/apollo-server-lambda/src/ApolloServer.ts#L106-L108), but it won't mirror the origin when using a wildcard as the origin (https://github.com/apollographql/apollo-server/blob/main/packages/apollo-server-lambda/src/ApolloServer.ts#L132). This means that it would fix installations which are specifying multiple domains, but not those specifying a wildcard (*).

It's a bit confusing 😃 It currently misses this to fix #223 completely.

[electerious]

The bug is indeed fixed by the PR. It seems that settings cors: true reflects the request origin. I previously thought that we need to do that manually. 🎉