Initialize() should not be called more than once

Add an assert checking that `FlutterDownloader.initialize()` is not called more than once, because that may end up registering plugins multiple times and crashing the app.

Related to fluttercommunity#154.

lib/src/downloader.dart

@@ -26,6 +26,8 @@ class FlutterDownloader {
   static bool _initialized = false;
 
   static Future<Null> initialize() async {
+    assert(!_initialized, 'FlutterDownloader.initialize() must be called only once!');
+
     final callback = PluginUtilities.getCallbackHandle(callbackDispatcher);
     await _channel
         .invokeMethod('initialize', <dynamic>[callback.toRawHandle()]);
@@ -385,4 +387,4 @@ class FlutterDownloader {
     _channel.invokeMethod('registerCallback', <dynamic>[callbackHandle.toRawHandle()]);
   }
 
-}
+}