Fix writing json numeric values

Working on #115

Ooui/JsonConvert.cs

@@ -76,14 +76,17 @@ namespace Ooui
 
             if (value is double) {
                 w.Write (((double)value).ToString (icult));
+                return;
             }
 
             if (value is int) {
                 w.Write (((int)value).ToString (icult));
+                return;
             }
 
             if (value is float) {
                 w.Write (((float)value).ToString (icult));
+                return;
             }
 
             w.Write (Newtonsoft.Json.JsonConvert.SerializeObject (value));