fix(timing): skip warning for falsy value

src/core.js

@@ -272,7 +272,7 @@ export function timing(
   trackerNames
 ) {
   if (typeof ga === 'function') {
-    if (!category || !variable || !value || typeof value !== 'number') {
+    if (!category || !variable || typeof value !== 'number') {
       warn(
         'args.category, args.variable ' +
           'AND args.value are required in timing() ' +

test/functionality/timing.test.js

@@ -79,6 +79,20 @@ describe('timing()', () => {
     );
   });
 
+  it('should not warn if value arg is 0, a falsy number', () => {
+    ReactGA.initialize('foo');
+    ReactGA.timing({
+      category: 'Test',
+      variable: 'Timing test',
+      value: 0
+    });
+    expect(spies.warn).not.toHaveBeenCalledWith(
+      '[react-ga]',
+      'args.category, args.variable AND args.value are required in timing() ' +
+        'AND args.value has to be a number'
+    );
+  });
+
   it('should warn if value arg is not a number', () => {
     ReactGA.initialize('foo');
     ReactGA.timing({