Handle a TTL of 0 correctly

When a TTL override of 0 is set, the override is overriden with the
policy due to using the || operator, which considers 0 to be false.

packages/apollo-datasource-rest/src/HTTPCache.ts

@@ -116,7 +116,7 @@ export class HTTPCache {
       return response;
     }
 
-    let ttl = ttlOverride || Math.round(policy.timeToLive() / 1000);
+    let ttl = ttlOverride === undefined ? Math.round(policy.timeToLive() / 1000) : ttlOverride;
     if (ttl <= 0) return response;
 
     // If a response can be revalidated, we don't want to remove it from the cache right after it expires.

packages/apollo-datasource-rest/src/__tests__/HTTPCache.test.ts

@@ -210,6 +210,21 @@ describe('HTTPCache', () => {
       expect(await response.json()).toEqual({ name: 'Ada Lovelace' });
       expect(response.headers.get('Age')).toEqual('10');
     });
+
+    it('allows disabling caching when the TTL is 0 (falsy)', async () => {
+      fetch.mockJSONResponseOnce(
+        { name: 'Ada Lovelace' },
+        { 'Cache-Control': 'max-age=30' },
+      );
+
+      await httpCache.fetch(new Request('https://api.example.com/people/1'), {
+        cacheOptions: (response: Response, request: Request) => ({
+          ttl: 0,
+        }),
+      });
+
+      expect(store.size).toEqual(0);
+    });
   });
 
   it('allows specifying a custom cache key', async () => {