postEncode expect all values to be strings

The ```sendTiming``` parameter ```time``` is of type ```int```, but internally type ```string``` is required. Im using 
```
      _analytics.sendTiming('LoadTime', stopwatch.elapsedMilliseconds);
```
but it crashes as ```postEncode``` in usage_impl.dart requires all map values to be strings, but ```'utt'``` is an integer. Therefore ```time``` should be converted to a string first.