Fixed an issue with getScreenshot (getPNGAsString)

We've been working with getting screenshots from IOS-Driver on real devices using:

```java
webDriver.getScreenshotAs(OutputType.BYTES);
```

We noticed that the image passed across the wire was rather large for a png and untimely discovered it was a raw tiff.

This change ensures that a base64 png string representation is passed instead.

From our perspective we have noticed significant performance gains (4 second image transfer to 0.5 second)