Add regression tests for issue reported in #2921.

Specifically, make sure to test all possibilities which might come in here.

packages/apollo-server-core/src/__tests__/isDirectiveDefined.test.ts

@@ -23,6 +23,19 @@ describe('isDirectiveDefined', () => {
     ) on FIELD_DEFINITION | OBJECT | INTERFACE
   `;
 
+  describe('When passed a DocumentNode', () => {
+    it('returns false when a directive is not defined', () => {
+      expect(isDirectiveDefined(gql(noCacheControl), 'cacheControl')).toBe(
+        false,
+      );
+    });
+    it('returns true when a directive is defined', () => {
+      expect(isDirectiveDefined(gql(hasCacheControl), 'cacheControl')).toBe(
+        true,
+      );
+    });
+  });
+
   describe('When passed an array of DocumentNode', () => {
     it('returns false when a directive is not defined', () => {
       expect(isDirectiveDefined([gql(noCacheControl)], 'cacheControl')).toBe(
@@ -36,6 +49,15 @@ describe('isDirectiveDefined', () => {
     });
   });
 
+  describe('When passed an array of strings', () => {
+    it('returns false when a directive is not defined', () => {
+      expect(isDirectiveDefined([noCacheControl], 'cacheControl')).toBe(false);
+    });
+    it('returns true when a directive is defined', () => {
+      expect(isDirectiveDefined([hasCacheControl], 'cacheControl')).toBe(true);
+    });
+  });
+
   describe('When passed a string', () => {
     it('returns false when a directive is not defined', () => {
       expect(isDirectiveDefined(noCacheControl, 'cacheControl')).toBe(false);