Use Object.fromEntries to convert Headers into a plain object.

As helpfully debugged and identified by @dsanders11 (Thank you!), this was a
mistake in my original implementation that incorrectly spread the iterable
`Headers` into the `headers` object of the response.

While I would have loved to create a regression test for this, there was no
place which was currently wired up to support handler specific options (like
`cors`), which is sometimes on the top-level constructor to `ApolloServer`,
and sometimes on `createHandler` or `applyMiddleware`.  I did try to futz
with it for a full hour to no avail.  I'm already aware of a number of
limitations with the current so-called `integration-testsuite`, and I'm
putting that on the list of things to address in an upcoming overhaul of
that testing infra.

That said, I've manually tested this and I think it's working properly.  An
upcoming release should hopefully confirm that.

Ref: https://github.com/apollographql/apollo-server/pull/2674/files#r288053103
Ref: #2718 (comment)

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

@@ -138,13 +138,19 @@ export class ApolloServer extends ApolloServerBase {
         }
       }
 
+      // Convert the `Headers` into an object which can be spread into the
+      // various headers objects below.
+      const requestCorsHeadersObject = Object.fromEntries(
+        Array.from(requestCorsHeaders),
+      );
+
       if (event.httpMethod === 'OPTIONS') {
         context.callbackWaitsForEmptyEventLoop = false;
         return callback(null, {
           body: '',
           statusCode: 204,
           headers: {
-            ...requestCorsHeaders,
+            ...requestCorsHeadersObject,
           },
         });
       }
@@ -167,7 +173,7 @@ export class ApolloServer extends ApolloServerBase {
             statusCode: 200,
             headers: {
               'Content-Type': 'text/html',
-              ...requestCorsHeaders,
+              ...requestCorsHeadersObject,
             },
           });
         }
@@ -180,7 +186,7 @@ export class ApolloServer extends ApolloServerBase {
             ...result,
             headers: {
               ...result.headers,
-              ...requestCorsHeaders,
+              ...requestCorsHeadersObject,
             },
           },
         );