[MOD-2407] iOS: apm.setMetadata(key, val) won't work in iOS if val is an integer

Hello, While doing some performance testing we came to notice that the setMetadata function seems not to log any integer values to the Appcelerator Dashboard in iOS, but this works fine in Android. However, the Appcelerator documentation says this function takes a String or Integer as its value parameter: https://docs.appcelerator.com/platform/latest/#!/api/Modules.Performance-method-setMetadata The version of the APM we're using is 3.1.0 for both Android and iOS, but all of these tests were executed in simulators/emulators. *Sample app/Test code:* https://propelics.box.com/s/jyzrh9l4gmz3hr1hf6veibtln8dedc4b *Steps to reproduce*: 1) In the sample app open index.js and observe how clicking in the label on the screen should add a username (JohnDoe) and some metadata (two strings and two integers) to the apm. 2) Make sure you modify the tiapp.xml with valid Appcelerator Platform GUID and Keys for the APM module to work. 3) Run the app on iOS. 4) When app launches, click on the "Click me!" label. 5) Look up for the app at http://dashboard.appcelerator.com 6) Look up for the user JohnDoe in the Performance tab, and select that user 7) Observe the metadata values, as only the string values will show there. 8) Additionally, run the application on Android. 9) When the app launches, click on the "Click me!" label. 10) Look up for the app at http://dashboard.appcelerator.com 11) Look up for the user JohnDoe (the one corresponding to Android) in the Performance tab and select it 12) All the metadata values will be there, including the integer ones. Thanks