Prevent Cache-Control header from being set if response has errors

[fabsrc]

Closes apollographql/apollo-cache-control#12

Adds a check for errors and for data before setting the Cache-Control header. This change is aligned with the logic in the apollo-server-plugin-response-cache package:

apollo-server/packages/apollo-server-plugin-response-cache/src/ApolloServerPluginResponseCache.ts

Lines 243 to 261 in 7db109e

	if (
	response.errors ||
	!response.data ||
	!overallCachePolicy ||
	overallCachePolicy.maxAge <= 0
	) {
	// This plugin never caches errors or anything without a cache policy.
	//
	// There are two reasons we don't cache errors. The user-level
	// reason is that we think that in general errors are less cacheable
	// than real results, since they might indicate something transient
	// like a failure to talk to a backend. (If you need errors to be
	// cacheable, represent the erroneous condition explicitly in data
	// instead of out-of-band as an error.) The implementation reason is
	// that this lets us avoid complexities around serialization and
	// deserialization of GraphQL errors, and the distinction between
	// formatted and unformatted errors, etc.
	return;
	}

[abernix]

Thanks for opening this!

Feedback addressed, one remaining question.

[abernix]

Wonderful. Thank you!