fix for safari

lib/model.js

@@ -24,6 +24,11 @@ var util = require('util')
  * But you really need to want it to trigger such behaviour, even when warned not to use '$' at the beginning of the field names...
  */
 function checkKey (k, v) {
+
+  if (typeof k === 'number') {
+    k = k.toString();
+  }
+
   if (k[0] === '$' && !(k === '$$date' && typeof v === 'number') && !(k === '$$deleted' && v === true) && !(k === '$$indexCreated') && !(k === '$$indexRemoved')) {
     throw 'Field names cannot begin with the $ character';
   }