Fix onDuration being called with null

Some players need onDuration to be called onReady (FilePlayer) and some need it onPlay (YouTube)
Instead of spreading onDuration amongst individual player files, here we just delay onDuration until onPlay if it returns null when onReady fires
Fixes cookpete/react-player#52

src/players/Base.js

@@ -9,6 +9,7 @@ export default class Base extends Component {
   static defaultProps = defaultProps
   isReady = false
   startOnPlay = true
+  durationOnPlay = false
   componentDidMount () {
     if (this.props.url) {
       this.load(this.props.url)
@@ -55,7 +56,10 @@ export default class Base extends Component {
       this.seekTo(this.seekOnReady)
       this.seekOnReady = null
     }
-    this.props.onDuration(this.getDuration())
+    if (this.durationOnPlay) {
+      this.props.onDuration(this.getDuration())
+      this.durationOnPlay = false
+    }
   }
   onReady = () => {
     this.isReady = true
@@ -68,6 +72,12 @@ export default class Base extends Component {
         this.play()
       }
     }
+    const duration = this.getDuration()
+    if (duration) {
+      this.props.onDuration(duration)
+    } else {
+      this.durationOnPlay = true
+    }
   }
   onEnded = () => {
     if (this.props.loop) {