Fix possible class cast exception parsing folder id

As per previous commit:
    commit 07eddf4

    Handle type-unsafe responses from LMS

    As we are not using customized parsers for our model objects, we simply
    parse incoming records to map. This parsing guess the type of the elements,
    so 2 becomes a Long, while "2" becomes a String.

    To handle this we check the type of the element in the map before using it.

Squeezer/src/main/java/uk/org/ngo/squeezer/service/SqueezeService.java

@@ -1518,7 +1518,7 @@ public void downloadItem(JiveItem item) throws HandshakeNotCompleteException {
 
         public Boolean randomPlayFolder(JiveItem item) {
             SlimCommand command = item.randomPlayFolderCommand();
-            String folderID = (String) command.params.get("folder_id");
+            String folderID = Util.getString(command.params, "folder_id");
             if (folderID == null) {
                 Log.e(TAG, "randomPlayFolder: No folder_id");
                 return false;