Fix DoubleClick Fast Fetch bugs around categoryExclusions and tagForC…

…hildDirectedTreatment (ampproject#7843)

* Fix incorrect parameter name for `tagForChildDirectedTreatment` in DoubleClick.

* Added unit test for tagForChildDirectedTreatment.

* Fixed the categoryExclusions bug in Fast Fetch DoubleClick.

* Removed parens in arrow functions to satisfy linter

extensions/amp-ad-network-doubleclick-impl/0.1/amp-ad-network-doubleclick-impl.js

@@ -94,7 +94,7 @@ export class AmpAdNetworkDoubleclickImpl extends AmpA4A {
     }
     const rawJson = this.element.getAttribute('json');
     const jsonParameters = rawJson ? JSON.parse(rawJson) : {};
-    const tfcd = jsonParameters['tfcd'];
+    const tfcd = jsonParameters['tagForChildDirectedTreatment'];
     const adTestOn = isInManualExperiment(this.element);
 
     const multiSizeDataStr = this.element.getAttribute('data-multi-size');
@@ -209,15 +209,13 @@ AMP.registerElement(
  * @return {?string}
  */
 function serializeTargeting(targeting, categoryExclusions) {
-  if (!targeting) {
-    return null;
-  }
-  const serialized = Object.keys(targeting).map(
-      key => serializeItem(key, targeting[key]));
+  const serialized = targeting ?
+      Object.keys(targeting).map(key => serializeItem(key, targeting[key])) :
+      [];
   if (categoryExclusions) {
     serialized.push(serializeItem('excl_cat', categoryExclusions));
   }
-  return serialized.join('&');
+  return serialized.length ? serialized.join('&') : null;
 }
 
 /**

extensions/amp-ad-network-doubleclick-impl/0.1/test/test-amp-ad-network-doubleclick-impl.js

@@ -209,5 +209,21 @@ describes.sandboxed('amp-ad-network-doubleclick-impl', {}, () => {
           '&dtd=[0-9]+$'));
       });
     });
+
+    it('handles tagForChildDirectedTreatment', () => {
+      element.setAttribute('json', '{"tagForChildDirectedTreatment": 1}');
+      new AmpAd(element).upgradeCallback();
+      return impl.getAdUrl().then(url => {
+        expect(url).to.match(/&tfcd=1&/);
+      });
+    });
+
+    it('handles categoryExclusions without targeting', () => {
+      element.setAttribute('json', '{"categoryExclusions": "sports"}');
+      new AmpAd(element).upgradeCallback();
+      return impl.getAdUrl().then(url => {
+        expect(url).to.match(/&scp=excl_cat%3Dsports&/);
+      });
+    });
   });
 });