set content type to text/html for playground (#2026)

BUGFIX: the playground page is currently being served without a content type

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TODO:

* [x] Update CHANGELOG.md with your change (include reference to issue & this PR)
* [ ] Make sure all of the significant new logic is covered by tests
* [x] Rebase your changes on master so that they can be merged easily
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CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ### vNEXT
 
+- `apollo-server-micro`: Set the `Content-type` to `text/html` for GraphQL Playground. [PR #2026](https://github.com/apollographql/apollo-server/pull/2026)
+
 ### v2.2.5
 
 - Follow-up on the update to `graphql-playground-html` in previous release by also bumping the minor version of the `graphql-playground-react` dependency to `1.7.10` — which is the version requested from the from the CDN bundle by `graphql-playground-html`. [PR #2037](https://github.com/apollographql/apollo-server/pull/2037)

packages/apollo-server-micro/src/ApolloServer.ts

@@ -130,6 +130,7 @@ export class ApolloServer extends ApolloServerBase {
           subscriptionEndpoint: this.subscriptionsPath,
           ...this.playgroundOptions,
         };
+        res.setHeader('Content-Type', 'text/html; charset=utf-8');
         send(res, 200, renderPlaygroundPage(middlewareOptions));
         handled = true;
       }