fix the issue when UIWebView javascript returns array

[UIWebview stringByEvaluatingJavaScriptFromString: ] only returns the correct data if the data is string
When executing player.getAvailablePlaybackRates(); it returns an array, so the quality array will never
be passed to the Objective C array.
fixed this issue by adding toString() on the javascript command, and parse the returned string int a NSArray
with [NSString componentsSeparatedByString:]