Fix <FormattedRelative> auto-updating with strings

This fixes an issue where `<FormattedRelative>` would re-render every
millisecond when `props.value` was a string (date timestamp).

Now, `props.value` is passed into `new Date()` first, so a string
value will be parsed.

Author: ericf

src/components/relative.js

@@ -69,7 +69,8 @@ export default class FormattedRelative extends Component {
             return;
         }
 
-        let delta = Number(props.value) - state.now;
+        let time  = new Date(props.value).getTime();
+        let delta = time - state.now;
         let units = props.units || selectUnits(delta);
 
         let unitDelay     = getUnitDelay(units);

test/unit/components/relative.js

@@ -199,6 +199,29 @@ describe('<FormattedRelative>', () => {
         }, 10);
     });
 
+    it('updates at maximum of `updateInterval` with a string `value`', (done) => {
+        const {intl} = intlProvider.getChildContext();
+        const date = new Date().toString();
+
+        // `toString()` rounds the date to the nearest second, this makes sure
+        // `date` and `now` are equal.
+        spyOn(intl, 'now').andCall(Date.now);
+
+        // Force scheduler by rendering twice with different props because
+        renderer.render(<FormattedRelative value={date} updateInterval={10} />, {intl});
+
+        // Shallow Renderer doesn't call `componentDidMount()`.
+        renderer.getMountedInstance().componentDidMount();
+
+        setTimeout(() => {
+            // Make sure setTimeout wasn't called with `NaN`, which is like `0`.
+            expect(intl.now.calls.length).toBe(1);
+
+            renderer.unmount();
+            done();
+        }, 10);
+    });
+
     it('does not update when `updateInterval` prop is falsy', (done) => {
         const {intl} = intlProvider.getChildContext();
         const date = new Date();