Compare Engine reporting's privateHeaders case-insensitively, as do…

…cumented.

The documentation for `privateHeaders`[[0]] suggests that it is
case-insensitive.  While that statement is true, and the incoming header is
lower-cased before checking it against the `privateHeaders` configuration,
it assumed that the headers in the `privateHeaders` object were specified in
lower-case.

This changes the comparison to lower-case both sides prior to determining
equality.

[0]: https://github.com/apollographql/apollo-server/blob/abb8dc58/packages/apollo-engine-reporting/src/agent.ts#L67-L70

Fixes: #2273

packages/apollo-engine-reporting/src/extension.ts

@@ -129,7 +129,10 @@ export class EngineReportingExtension<TContext = any>
           // We assume that most users only have a few private headers, or will
           // just set privateHeaders to true; we can change this linear-time
           // operation if it causes real performance issues.
-          this.options.privateHeaders.includes(key.toLowerCase())
+          this.options.privateHeaders.some((privateHeader) => {
+            // Headers are case-insensitive, and should be compared as such.
+            return privateHeader.toLowerCase() === key.toLowerCase();
+          })
         ) {
           continue;
         }